So you have a neat Arduino project going on. Wouldn't it be nice to add Internet connectivity and do it on the cheap as well? This Instructable will show you how to add Internet connectivity in the form of an Ethernet interface for a few dollars and in less than half an hour. More info at: http://hwstartup.wordpress.com/
Apart from your Arduino, you need a read-made Ethernet module. You can easily get these on eBay for as low as 10$. Just search eBay for ENC28J60 module. In addition, you need to have a bit of electrical wire, a soldering iron and some soldering 'tin'.