Taken from Chrome Book Ratings
Connecting a Chromebook to Wired Ethernet/LAN
Even though Chromebooks have built-in Wi-Fi and wireless 3G internet adapters, you’ll find that there’s no wired ethernet port to be found on their compact chassis. But don’t fret, there’s a way! According to the documentation from Samsung and Google, most USB ethernet ports should work with Chromebooks just fine. Good USB ethernet network adapters can be found today for pennies (well, in computing terms), and will allow you to plug in a standard RJ45 ethernet jack and connect to a wired network/LAN. So, even if you have only a wired router at home, you’ll be just fine.
Update: There are a ton of options for USB ethernet adapters, of varying quality and compatibility. Personally, I recommend this Cisco-Linksys USB Ethernet Adapter for build quality/brand reputation, lower price, and compatibility with a wide range of devices.
Also on Google's support website, see point 4: Initial setup - Chrome OS Help