One more vote for the Monoprice adapter. Works great (just make sure you install the latest driver) and I've already use it for some time now in various network environments for a lot of hours. Not a single problem so far.
Also, it gives you higher speeds than apple adapter (and all other 100Mbit adapters out there) since it supports gigabit ethernet (you'll not get a full gigabit speed of course because of usb 2.0 limit, but significantly higher speeds than the 100 Mbit adapters). Highly recommend it at least until there are any thunderbolt ones.