Music Friend (Fallen Sprite) [official page] is pretty much what its name suggests: a musical little friend. Made by Brenda Neotenomie (her behind the soundtrack to Porpentine's With Those We Love Alive), it's a free musical diorama of a sprite (from Lily Zone's Crypt Underworld) gently rocking out to soothing music on a rotating planetoid. I've enjoyed having it open in the background playing music, occasionally visiting to bang out a quick tune on the interactive bits - jabbing a cupcake, mostly.
Neotenomie calls it "a cute drowsy musical toy", which I'll go along with. I'm not sure what more to say. You can play with it. You can listen to it. You can listen to it and play with it. I like it. And it's pay-what-you-want with no minimum. And on Windows, Mac, and Linux. It's here.
Anywho, I know how hard it is to get people to try even something free, so here's a video as further enticement. It's a few minutes of how I see Music Friend, mostly in the background as I do something in another window, with brief visits to chime away. I do apologise for the blur, but Music Friend runs at 480x360 (I did say it was little) and YouTube really chews up low-res video.