I’ve mentioned Helium a couple of times as my favorite music management app. It still has a few flaws but the developers are working on it:
BPM automatic calculation (great for Tracktor DJs, this feature doesn’t work well right now)
External data queries to retrieve artist info, style, tone and more from Allmusic.com and other sources.
Other interesting related apps:
MP3tag can be used to rename files based on their tags, which is useful if you’re taking files with long truncated names from CD-R to hard drive. (Helium does that too but maybe you don’t care to buy it.)
The Godfather handles richer external queries than Helium right now.
Winamp 5 is said to be much better than the bloated 3.0.
04/02/04 update: key data (as in B#, Gb) is missing, some DJs subscribe to Camelot Sound Services to know what tracks will mix harmonically.