Warzone 2100, it’s one of the best RTS games I’ve ever seen. It recently (2004, so maybe not so recent) has gone open source through means of the GNU General Public License (version 2 of it). Warzone is now being maintained by the Warzone2100 Resurrection Project.
This group has quite an active developer commmunity - lets just say their developer’s mailinglist provides more mail for my inbox than the regular spam (although that probably says more about my spamfilters). Compiling it however isn’t yet very straightforward on Windows and the documentation provided along with the Subversion repository isn’t that up-to-date. So for those reasons I’ve compiled the latest version of the 2.0 branch (not the main developer tree, I’m still working on compiling those). I’ve packaged the executables into a nice installer package. There are two variants the “release” or non-debug version and a debug version.
Update: These two installer packages are removed (the branch 2.0 variant). Instead I’ve made the mainstream developer version available (also known as the “trunk”) look here for that one: Warzone 2100 developer version.
Update 2: You can find Warzone 2100 2.0.5 here.