Photoshop, sometimes GIMP when I'm using Linux, but honestly Photoshop is the best for producing web design mockups. Illustrator is good if you have the skills, but I never got into creating vectors and using Illustrator so I stick with Photoshop.
Yes! Thrive Themes and their suite of plugins are both affordable and user-friendly, even if you aren't deep into WordPress. A great option, I use their tools on all my projects!
Pixate can be for generating spontaneous, fun prototypes for iOS and Android, an app that has been created. What sets Pixate aside from other methods I Have used is its drag-and-drop animation and discussion panel.
There are several web designing tool available online. Moist of it are very amicable and easy to use. Somehow, if you want the best quality performing tool, read on the reviews and other opinions from those who have used each product.