Slugathon is a free (licensed under the GPLv2) clone of the classic Avalon Hill boardgame Titan. The focus is on playing against other humans across the Internet.
It’s playable, but there isn’t a public server up yet. (Soon!) If you want to give it a shot, check out the instructions in README.txt.
Slugathon is written in Python, using Twisted for the networking and PyGTK for the GUI. It works on Linux, Windows, and Mac OS. (It’s a bit painful to install on Windows and Mac OS though.)
Here are screenshots of the masterboard and a battle map: