MediaMonkey
MediaMonkey is a Windows-based media player/library, with built-in file tagger & organizer, CD ripper/burner (limited speed on free version), transcoding tool, replaygain tool, and lots others, all in an integrated user interface natively themed as a media library, quite unlike other media players.
It maintains its library in a database that is compatible with Microsoft Access database format, and as such, the database entries can be changed by using Access or other compatible programs. However, it also stores most of its information (if supported) into the media files' tags (ID3v1, ID3v2, Ogg comments, WMA, APE2, and WAV tags are supported), thus ensuring usability with other media players.
MediaMonkey also provides auto-tagging/auto-renaming support, in which tag information are deduced from file's path and name, or reversely, renaming (and relocating) files based on their tag information.
Finally, MediaMonkey also support integration with various portable DAPs, including Apple iPod, iRiver, and Creative Labs devices.
Supported Formats
Additional formats may be supported through Winamp input plugins (see below)
Extensibility
MediaMonkey can use most Winamp's input, output, DSP, and visualization plugins.
In addition, MediaMonkey provides scripting interfaces, so it is scriptable (built-in support) and also controllable by external applications.
Price
The free version, is, well, free. The Gold version (which adds, among others, automatic/periodic scan of "watched folders") is 29.95 USD.