Foobar 2000 is pretty cool. It comes very simple package.
I mainly use it for it's converter, so I can mess around with audio tracks extracted from BluRay and DVD movies. I can decode both AC3 5.1 and DTS 5.1, and convert to AAC 5.1 for mp4's.
I just started ripping CD's and various audio in WAV and mess around with different encoding options.
I find the help with it limited, but you can check out : Hydrogenaudio Forums -> foobar2000
You can manipulate the panels and columns with it, and pretty much create your own skins.
You can get plug-ins for mass tagging, decoders for obscure audio formats, pretty much every format I have ever heard of.
I like it, just haven't fell in love with it yet. I still have WMP kickin most of the time.