I use iTunes on my Mac to manage my music library(currently 475GB), as I haven't found anything better for the job, especially for managing playlists. Plus, with my Airport Express on the stereo downstairs, and the Remote app on my iPod, I can control my music from anywhere in the house. I am a bit surprised there aren't better music players for the Mac.
On my PC, I still use Winamp and Shoutcast to troll for new music. I've been using Winamp since the '90's and it was responsible for my finding a ton of great ambient music. Nothing better than listening to Tetsu Inoue and watching one of Winamp's waterfall plugins. Sentimental, I know. Oddly, I've never installed iTunes on the PC.