GNU Aspell: I thought this was kind of interesting. Given my ignorance, I’m sure it’s been around for years.
GNU Aspell is a Free and Open Source spell checker designed to eventually replace Ispell. It can either be used as a library or as an independent spell checker. Its main feature is that it does a superior job of suggesting possible replacements for a misspelled word than just about any other spell checker out there for the English language.
I searched for a while but couldn’t find an actual sample of using it for server-side spell checking (either checking user’s content text or checking search terms). If you know of an implementation with code samples, speak up.