Windows/Mac/Linux: There's no shortage of great desktop music players out there, but if you just want something simple that integrates other services better than iTunes, cross-platform Clementine might be just for you.
Amarok 2 has been out for a while, but a lot of Linux users still prefer the less cluttered Amarok 1.4. Now, users of any operating system can get in on the action with the Amarok 1.4-inspired Clementine. It may not be as full-featured or unique as something like MeidaMonkey or foobar2000, but more casual listeners unhappy with iTunes' lack of Last.fm or FLAC support will enjoy it.
It's a pretty basic playlist-based player from which you can browse either a music library or just your hard disk. It supports a number of formats, including the oft-ignored Ogg Vorbis and FLAC, as well as a few different internet radio sources like Last.fm, SomaFM and Magnatune. It has support for notifications from both libnotify (on Linux) and Growl (on OS X) built in too. The only issue I had with it was that it didn't support the embedded cover art on any of my files — but its built-in cover art manager took care of that quickly by re-populating them all with info from Last.fm.
It's still in early stages, so it may become more feature-filled as time moves on, but for now, it fits perfectly into the niche of people that don't need a superpowered music player but would like built-in support for some of the things that you need to install separately from something like iTunes (such as Last.fm, Growl and FLAC).
Clementine is a free download for all platforms.