Windows/Mac only: Tag your iTunes music by mood with the freeware, colour-based tagging utility Moody. While your music is playing, just pick a colour for the song with Moody's 16-color scale (sad to happy, calm to intense). Once tagged, Moody writes the mood to the comments of your song's metadata (it'll look something like MoodyC3). You can either use that metadata to create mood-based smart playlists or just use Moody to fire up a playlist based on your mood. It may sound a bit tedious, but if you put Moody in quick tag mode, you can tag a lot of music pretty quickly. Moody is freeware, Windows (with .NET) and Mac OS X only.