If you're a DJ or music aficionado with a large collection of CDs and/or LPs, you know that keeping track of them can be a chore. VisiTrax Lite is a free librarian that helps you catalog your tunes (automatically for CDs by looking them up in an online database). Then you can easily find them using a number of criteria, including the usual artist and song name--but also extending to media format, era, BPM (beats per minute), and even how long the song lasts.
VisiTrax Lite's interface is no-nonsense and easy to use. It's not as pretty as say iTunes, but it'll get the job done quicker. It also has some nice I/O options including CSV export to spreadsheets as well as uploading to the http://www.djintelligence.com/ and http://www.wedj.com/ DJ booking sites. Note that VisiTrak is not a player, though. It only catalogs and is therefore of limited use for MP3 collections.
The Lite version of VisiTrax is fully functional, but limited to 100 CDs, LPs, etc. or 1000 tracks. The Standard version, which does away with the limit, is available for $30.