In almost every strategy, management or sim game I play, I will immediately turn off the music which comes with the game in favour of my own. That means that Steam Music Player sounds like a good idea to me even if I long ago abandoned mp3s in favour of streaming. The built-in functionality, which lets you browse your music library and control playback from in-game using the Steam overlay, has just left beta after its initial announcement back in February.
To celebrate, Valve have made the soundtracks for some of their games freely available to those who own the associated games, including Half-Life, Half-Life 2 and its Episodes, Portal, Portal 2, and the Dota 2 documentary Free to Play.
Those base games are also 75% off until October 1st, on the off chance you don't already own some of the best and most successful PC games of all time.
Valve are still looking for feedback on what features they should add to the service next on the Steam Music group. Given how bad iTunes is and how antiquated Winamp feels, it can't hurt to have Valve doing something of their own.
Anyway. I'll leave you with the only song you'll ever need.