Kelly Bailey's music in Half-Life 2
 

If you have played Half-Life 2 you will know what I mean. Very upbeat and fast Electronic music that has a good beat. I love it and sometimes find myself playing Half-Life just to listen to the music.

I downloaded the soundtrack to the game after playing it. It's pretty good. It works a lot better within the context of the game then as a stand alone listen.

You get it free if you buy audiosurf from steam, which is actually a very good game in itself. Well worth it.