It would be nice if you can share your experience on the pros and cons of each of them. Personally I only know Google Desktop and Beagle, and I haven't really used them. I mainly store my files on Windows (and use its integrated indexed search) but I'm trying to see if I can switch over to Linux.
Also, can any one of the search indexer run without X? Does any of them provide an API for search queries?
Edit: At a minimum I would like to be able to index file metadata for documents (including PDF, MS Office) and music. Would be nice if it can peek into compressed archives too :)