Software that turns a mac into a router, essentially. Created by sustainable networks. It basically puts a gui on not-normally-activated features in Open Transport. It lets you do things like assigning multiple IPs to the same computer, and-- more importantly-- ip masquerading. You can use IPNetRouter (as i do) as a simple way to share a single IP (say, from a DSL modem) transparently among several computers, or if you feel like it you can set up a full-blown firewall with DHCP for the computers behind it and filtering and NAT and inbound port mapping and all those other nifty things. It is, of course, not a good idea to put a firewall on a mac, since the mac has no memory protection and is crash-prone, but as long as the mac is not being regularly used while IPNetRouter is on it it should be perfectly safe. I can't comment authoritatively on how IPNR does in terms of performance when compared to the equivalent (free) linux/unix utilities, but i can tell you the learning curve on the user interface for the administrator is not nearly as steep.

IPNetRouter is $80 (although there is an educational discount) but has a full demo with no features crippled, so you can try it out and make sure it works for what you need it to before you decide to use it. I dunno if you would really want to use it for anything critical, but for a quick-fix it works beautifully.

IPNetRouter is not available for Mac OS X, and i do not believe they plan for it to be in the future. You can get the same features, however, with a combination of natd and ipfw on the terminal command line. Some third-party GUIs exist for this.