Google Web API's
I just stumbled across "Google Web API's":http://www.google.com/apis/ . Looks to be some very cool stuff. I hope to play with them soon.
The service is free and once you sign up, you're assigned a license key that gets you 1000 queries a day.
It looks like you can issue a general search, duh, and get the results back in XML so you can traverse and process the results any way you'd like.
Second, you can get a binary cached representation of any URL, so long as Google's crawlers visited the page.
Third, you can get spelling corrections, just like google does, when you make a query. This is useful if you've mistyped a word or words in your query and want to know if an alternate spelling will yield the search results you were really looking for.
Finally, you get a lot of options for all of these query types. Most or all of these options are available through Google's standard browser interface, but perhaps you can envision a better interface or an interface that conveniently compares results when various filters are applied.
Yum.