Winamp - This is the definitive MPEG Layer I, II, III, module and other sound format player for 32-bit Windows. It has a nice user interface, good playlist facilities and is being developed constantly. It's so good I actually paid money for it ($15!).
Maplay for Win32 - Maplay is a freeware MPEG Layer I, II and III player for Windows '95 & later. It supports playlists and has quite a nice interface. It's available in versions for 486's or Pentiums. It seemed a bit unstable when I tried it, but it's probably improved since then.