This is a very simple 2 speed transmission that I think anyone with a little bit of experience with Legos can build from a picture.
I get this idea from Sariel's* 4 speed transmission. I wanted to buy all the parts necessary to build his, but decided against it because I thought I could build my own, simpler, smaller version.
1st gear has a 3:1 gear ratio, which means the driver gear (the 8 tooth) has to spin 3 times to make the follower gear (the 24 tooth) spin once.
2nd gear has a 1:3 gear ratio, which means the driver gear (the 24 tooth) has to spin 1 times to make the follower gear (the 8 tooth) spin 3 times.
Sariel's Youtube : http://www.youtube.com/user/searme/
His Website: http://sariel.pl/