After doing some more looking around, the choice I would make is clear: fwbuilder, also known as Firewall Builder. I've heard of it before, but looking at the website I always thought it was a commercial product. It turns out, though, that it is free and open source for the linux program, and only the MS Windows and Mac programs are closed source and paid for.
It works like Firestarter in that you run it on your workstation, and then it transfers the configuration to the target firewall using ssh and scp. I like this because it means you don't have to install anything extra and there no extra running daemons on your firewall.
Firestarter. I have not used it (yet), but it looks the most interesting, and the one I would try out first.
From the introduction page:
Firestarter can be installed onto individual servers and managed graphically over SSH or using the shell.
That or ufw, possibly with Gufw, which is the standard on Ubuntu.
Update: it looks like there is Turtle Firewall, which is the standalone version of the iptables frontend included in Webmin. And is still maintained, unlike Firestarter.