What's your budget? As dstevens suggested, a routher w/ NAT is an alright solution if you have a low budget. If you're looking for a professional solution, I would recommend a Cisco PIX firewall. It's a lot safer than a cheap box with NAT enabled, though it's also significantly less expensive. If you're looking for a cheap solution and aren't able to set up a Linux box as jcdux recommended, I would personally recommend the Linksys EtherFast Cable/DSL Firewall Router. You'll get the most for your money with that, and, in my experience, Linksys has been quite reliable.

AJ