The ubiquitous and very wonderful Google has released a beta-preview of Google Desktop which allows you to search not only the usual Google places, but also your own computer. At this point, it appears that the files that it will search are restricted to documents created by Microsoft applications (Word, Excel, OutLook), as well as AOL instant messages, and plain text files. There is a good introduction to installing and using the tool here.

Also a graphics indexer called Picasa will index graphics files in multiple formats.