Here is a heck of a deal. This Arduino Ethernet Shield uses the WizPro 5100 like the official Arduino shield:
If this $10 shield works the same, it is a very cheap way to get internet connectivity out of an Arduino. If you get one, let me know how it works for you.
The shield I used for my Ethernet experiments was made by Sainsmart and it runs about $20:
And for those who don’t want to wait for mail order (from China?), you can get a $20 Seeed ethernet shield at your local Radio Shack: