I use Windows Media Player as my default.

I'd suggest an interesting alternative if you want to do a bit of disc jockey work. There's a freeware program called T-Player. It's a smallish program, but it makes mixing songs pretty easy - beyond the typical setting up of play lists, it has a complete randomizer for all music files on your computer (that you want it to play) regardless of format. It also will mix from song to song - so instead of the typical pause beween song it automatically goes into the next song.

On top of that it's quite user friendly. I act as dj at dances at my church and just bring my PC along to plug into the sound system there. 3000+ songs at my disposal, all easily at hand with T-Player.

It's not perfect - for instance, it separates all songs into those be a different performer whether you want it to or not. This may not sound like a problem, but it's been a staedy bit of work changing the id files of the mp3s to that the performes names are all spelled correctly and follow the same convention. For example, I had songs listed under:

Crosby Stills Nash and Young
Crosby, Stills, Nash and Young
Crosby Stills Nash & Young
Crosby, Stills, Nash & Young
Crosby Stills and Nash
Crosby, Stills and Nash
etc.

By the way, if you want to organize your mp3 files so that the id tages are all nice and neat, spelled correctly, etc. I use Godfather and am very happy with it. It is also freeware.