We actually have two - one we use at nighttime, and one for naps.
The one we use at night has softly glowing lights and plays soothing nature sounds and lullabies. It's called Tiny Love My Nature Pals. During the day we use Bright Starts Soothing Safari Mobile (no lights, just a regular mobile). Both play music for a decent amount of time. Also, both of them have an option of removing the mobile arm and having the music box stay attached to the crib so that older babies can still enjoy the music without you worrying they'll get tangled in the mobile.
Another option is a soother. We have the Baby Einstein Sea Dreams Soother that has lights, motion, and music. It plays for 20 (30?) minutes, and the music gradually gets softer the longer it plays to lull baby to sleep. It also comes with a remote control, although we've never used it.
I couldn't decide on just one thing for the crib, hence the long list. The crib looks pretty ridiculous with all this stuff hanging off it, but we get good use out of all of it, so oh well. :O) Hope that helps!