Some registrar have API access. The ones I use (eNom, DomainDiscover) allow you to hit about 1 per second using the API.
I used to have a sweet script that could call a Google docs spreadsheet, grab the next domain in the list, and hit up my registrar to try to register it.
Very handy for the drop timeframe each day, but I ended up spending way too much money on domains, so when it broke it was a blessing.
There are a few people selling similar windows desktop apps that use the registrar APIs, just google them. I think I saw one that was around $90, haven't tried it.