I'll preface this with saying that a lot of the info I'm going to be writing about has been written about. In fact, given the volume of information in existence, it is near impossible to have a completely new idea; even if it did germinate exclusively from your brain.
Now, I'd like to expose some ways I have enjoyed getting kick ass links, content and profited over the years.
To give some background: Most of my time in the industry, I have have been an affiliate or a consultant. I was a product provider for a while, but that didn't end up so well. Now I'm making my second attempt at moving up the food-chain, but have run into issues (personal shit that is interfering with business) So most of my stuff comes from the perspective of a consultant or an SO-oriented affiliate guy. Many of these topics have been discussed, but I'd like to give a little depth to them, since the nuances can make or break the applicability of a technique:
1- (content + links) Pre-Copyright Content
It turns out that most books published before 1923 are, for the most part, out of copyright. This means, if you scourge amazon, you will likely find books from before that date that are not popular and are on the topic of your niche (assuming your niche isn't something modern). You can buy these books, scan them, OCR them and then publish them on your website (or get a college kid to do it for you) This can give you tens of thousands or even hundreds of thousands of words of very low cost, very high quality content.
It also happens to be that there are many websites regarding old books, and aficionados who love to read them. Contact them and tell them about your book, and offer them that they can give it to their users for free; in exchange of a link.
Wikipedia is also quite open to including citations on a topic that are referenced from a book
Last, many larger sites in different niches like to seem more legitimate by linking to reading material on the topic.
Legal notice; to further find out about the legality of this technique read here:
LibraryLaw Blog: The myth of the pre-1923 public domain
2- (content) Transcribing youtube videos
Many times, there is top notch content in your niche in the form of youtube videos; be it reviews, news or general banter. For a few dollars, you can upload it to speechpad.com and have hundreds of words of top notch content. To not get in trouble, I tend to give a link back to the youtube video and, if the youtuber has a website, also to his/her website. I don't normally use this for money sites; but it is GREAT for building up powerful web 2.0 sites. I haven't gotten in trouble with this technique, but I know it is in the gray area; use with caution.
3- (content) Setting up writing contests
This is particularly good for niches with difficult to get content. I did it for one of my casino affiliate sites. I created a 'writing contest' website, totally unrelated to casino. The idea was you had to write a story based on a picture posted, a different one posted each month. First picture I used was totally unrelated to casino. I got the site listed on a TON of free contest sites and writer forums (they will link to you for free and some have MASSIVE traffic). After that, even though the website never mentioned gambling, it always had a picture of people at a slot machine or at a table game. I got some grade A+ articles written, basically for free.
In terms of prizes, I'd make the 'main prize' something very attractive, but always free myself from having to give it by including in the terms of the contest that the main prize was ONLY given if a large number of contestants participated, which I knew we would never reach. The secondary prizes were free chips at various online casinos; they cost me nothing and actually netted me a profit from the ones that ended up depositing.
4- (links) Find expired domains and 301 or resurrect the site
If you take a simple and free program like xenu link checker, and start going over a number of the top ranking sites in your niche, I'm sure you will find at least a couple of links to domains that are expired and available for registration.
You can then 301 these domains to you main site/inner pages, or can re-create the site via archive.org and have it link you. I've scored some really awesome buffer sites with this method (I tend to prefer to resurrect them)
Sometimes the domain is taken, but I have purchased them this way also (depends on the new owner, most want crazy amounts for their domains, but some will sell it in the low 3 digits)
BTW, when you resurrect, don't include your links right away, I like to bring it back to life exactly as it was, so that Google views it as 'back online', only after a month or two and adding an article or two do I drop my link. And I tend to prefer to make the link from a 'banner' so it looks like advertising and not some SEO bullshit.
Now, I'd like to expose some ways I have enjoyed getting kick ass links, content and profited over the years.
To give some background: Most of my time in the industry, I have have been an affiliate or a consultant. I was a product provider for a while, but that didn't end up so well. Now I'm making my second attempt at moving up the food-chain, but have run into issues (personal shit that is interfering with business) So most of my stuff comes from the perspective of a consultant or an SO-oriented affiliate guy. Many of these topics have been discussed, but I'd like to give a little depth to them, since the nuances can make or break the applicability of a technique:
1- (content + links) Pre-Copyright Content
It turns out that most books published before 1923 are, for the most part, out of copyright. This means, if you scourge amazon, you will likely find books from before that date that are not popular and are on the topic of your niche (assuming your niche isn't something modern). You can buy these books, scan them, OCR them and then publish them on your website (or get a college kid to do it for you) This can give you tens of thousands or even hundreds of thousands of words of very low cost, very high quality content.
It also happens to be that there are many websites regarding old books, and aficionados who love to read them. Contact them and tell them about your book, and offer them that they can give it to their users for free; in exchange of a link.
Wikipedia is also quite open to including citations on a topic that are referenced from a book

Last, many larger sites in different niches like to seem more legitimate by linking to reading material on the topic.
Legal notice; to further find out about the legality of this technique read here:
LibraryLaw Blog: The myth of the pre-1923 public domain
2- (content) Transcribing youtube videos
Many times, there is top notch content in your niche in the form of youtube videos; be it reviews, news or general banter. For a few dollars, you can upload it to speechpad.com and have hundreds of words of top notch content. To not get in trouble, I tend to give a link back to the youtube video and, if the youtuber has a website, also to his/her website. I don't normally use this for money sites; but it is GREAT for building up powerful web 2.0 sites. I haven't gotten in trouble with this technique, but I know it is in the gray area; use with caution.
3- (content) Setting up writing contests
This is particularly good for niches with difficult to get content. I did it for one of my casino affiliate sites. I created a 'writing contest' website, totally unrelated to casino. The idea was you had to write a story based on a picture posted, a different one posted each month. First picture I used was totally unrelated to casino. I got the site listed on a TON of free contest sites and writer forums (they will link to you for free and some have MASSIVE traffic). After that, even though the website never mentioned gambling, it always had a picture of people at a slot machine or at a table game. I got some grade A+ articles written, basically for free.
In terms of prizes, I'd make the 'main prize' something very attractive, but always free myself from having to give it by including in the terms of the contest that the main prize was ONLY given if a large number of contestants participated, which I knew we would never reach. The secondary prizes were free chips at various online casinos; they cost me nothing and actually netted me a profit from the ones that ended up depositing.
4- (links) Find expired domains and 301 or resurrect the site
If you take a simple and free program like xenu link checker, and start going over a number of the top ranking sites in your niche, I'm sure you will find at least a couple of links to domains that are expired and available for registration.
You can then 301 these domains to you main site/inner pages, or can re-create the site via archive.org and have it link you. I've scored some really awesome buffer sites with this method (I tend to prefer to resurrect them)
Sometimes the domain is taken, but I have purchased them this way also (depends on the new owner, most want crazy amounts for their domains, but some will sell it in the low 3 digits)
BTW, when you resurrect, don't include your links right away, I like to bring it back to life exactly as it was, so that Google views it as 'back online', only after a month or two and adding an article or two do I drop my link. And I tend to prefer to make the link from a 'banner' so it looks like advertising and not some SEO bullshit.