Desktop search - software you install on your computer which "spiders" and indexes documents (files) and/or email on your computer's hard disk(s). After indexing, you can search your system using keywords or portions of the filename and quickly find and open files, documents, and emails. The software runs in the background, watching for new files/email added to your system (or changes to your files).
Slate article/review on several Desktop Search applications
Semi-Off-Topic: Lookout - a plugin for Outlook that can search email and files in a "desktop search" fashion
Off-Topic: Google releases v2 of Picasa, the free image organizing (and now editing) software application.