Fast, light-weight, small media manager/player.
Foobar2000 is a really awesome media manager. It is VERY simple and light-weight. It's a great replacement for colossal iTunes. Foobar2000 is much faster at doing everything than iTunes. The application markets itself as being entirely customizable, a standard it holds up to. Check out lifehacker's guide to "Rolling your own killer audio player with foobar2000."