Some people don't know this oh-so-cool plugin in Winamp, the automatic "Winamp Playlist Generator". It is powered by Gracenote (a big music database) and allows you to create a playlist similar to any tracks of your library, in two clicks and a few seconds (Well, it also needs hours of library scanning).
You can do similar things in foobar. Well, the following is the closest I found.
1. Install the component foo_scrobblecharts.dll
It relies on the Last.fm database, but does not need a last.fm account.
[I don't know how gracenote similarities are calculated. Last.fm similarities are calculated essentially based on user's listening habits, and correlations]
2. In the context menu of any artist, you get a "Last.fm" element, with several options:
[artist]'s Similar Shuffle Playlist : generates a playlist of tracks similar to that artist. This is the closest I found from the Winamp plugin.
There are a few more features:
[artist]'s Similar Charts Playlist: Generates a playlist of tracks similar to the artist, ordered by decreasing order of popularity in the Last.fm (weekly) charts.
[artist]'s Charts Playlist: Generate a playlist of tracks by this artist, ordered by decreasing poplarity in the Last.fm (weekly) charts.
In the context menu of a selection of tracks in a playlist, you can order them by decreasing order of popularity.
NB: For all this, you need an active internet connexion.
If you want to listen to a track-based playlist, listen to Pandora if you live in the US (I don't :'-( )
If you want to listen to an artist-based playlist, you can listen to last.fm.
But this plugin is also a good way to discover you library.