Some of these are slightly feminine but any one can be made to suite a little boy.
This one can be found at Gather and Nest. I love it because you don't need to start completely from scratch.
I found this mobile over at Bugs and Fishes. The butterflies can easily be changed to owls, foxes, or other furry little creatures.
I really like how simple (and cheap!) this one is from the Artful Abode.
Spool might just have my favorite mobile yet:
