Google just released Google Desktop for Mac. This adds searching and indexing of:
- PDF, TXT and HTML files
- email for Apple Mail and Microsoft Entourage
- iChat chats
- Word, Excel, PowerPoint files
- Address Book contacts
Spotlight also supports searching these thing, but Google Desktop also search the text of web pages that you have visited. Also, you can search text in older versions of your document, even if they have been deleted.
I have installed it and it is working well, though the index hasn't completed yet.