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There are many choices here, ranging from simple NAT boxes costing around $50 to full firewall-type devices.
Windows 98SE, ME, 2000 and XP all have connection sharing (NAT) features. These are effective but obviously require the routing PC to be powered for others to have access. XP's connection sharing / firewall is UPnP aware -- allowing UPnP aware applications to operate properly despite the use of NAT incompatible techniques, e.g. Windows Messenger and Netmeeting.
There are also many 3rd party products which offer additional features, e.g. Kerio's Winroute product line.
The Linux kernel has NAT functionality built in in the form of "ipchains" in the 2.2 kernel and "netfilter" in 2.4. Netfilter offers stateful packet inspection, packet matching on a wide range of fields and various modules for additional functionality. To perform NAT, one would typically use the masquerade target, although more complex configurations are possible. A Linux box based upon the 2.4 kernel makes for an extremely versatile and configurable firewall / NAT device. For UPnP support, check out this sourceforge project page.
4. Other OSs
Other Unices, such as the BSD variants and Solaris, generally have features similar to those in Linux.