Can your MP3 collection afford to be without this app? Currently only available for Linux – would make a nice Win32 PHP-GTK project, no?
DigitalDJ is an SQL-based mp3-player frontend designed to work with Grip. When Grip encodes mp3 files, it places all song information into an SQL database. DigitalDJ then uses this information to create playlists based on various criteria.

• Manage a database of CDs in MP3 format
• Interacts with Grip to add song information as you rip
• Create playlists based on song properties such as Artist/Disc/Genre/Tempo
• Play songs via an external MP3-player
• Condensed mode with small screen footprint and scrolling display
• Nifty BPM-O-Matic beats-per-minute calculator
• Keeps track of number of plays and last play for song/disc/artist