We host websites in a shared hosting environment where Microsoft SQL Server full text searching is not allowed. We would love an ASP.NET API that allowed similar functionality to get around this restriction.
We can't easily install software on the shared servers, so the API would have to be written in ASP.NET.
SQL "like" queries are our alternative and they are fast enough (our websites never exceed more than 50Mb of text) but they don't rank results well, have a dictionary, do stemming etc
For this type of circumstance I'd rely on Google and create a proper sitemap. You can integrate google search right into your website too with Google SiteSearch.
If you need more control over full-text search, you can use features of the RDBMS to support this. You don't say which brand of RDBMS you're using. I assume it's likely Microsoft SQL Server if you're using ASP.NET.
See the docs for Full-Text Search at MSDN.
For other brands of RDBMS, see my answer: http://stackoverflow.com/questions/1684356/how-best-to-develop-the-sql-to-support-search-functionality-in-a-web-application/1684596#1684596
Lucene is what we were looking for http://incubator.apache.org/lucene.net/