Hey, I'm up early - or in bed late. Anyway, I came across a few open source flight combat simulators last night and thought I'd comment on them. I thought the OS flight combat sim scene was barren, but that is definitely not the case.
The main protagonist in this FLOSS game genre is GL-117. It's been stable and polished for quite some time now. There is vaunted development on a v2 of the game, but nothing tangible to speak of. It is more an arcade game than a simulation but packs of fun nonetheless!
RedShift is inspired by GL-117, so will be another arcade flight combat game. It's early days and in the progress of a rewrite from C to C++ but the author is optimistic that he'll be able to release an update in a month or so.
Flying Guns is a WWI flight combat simulation game where you can engage as many as 100 AI planes at a time. It's intrigingly written in Java and does look very, very cool & fun... but the webstart prototype version didn't work for me. :-(
If you want something a little more futuristic then you'll be wanted to take a look at Thunder & Lightening. Development is very active lately. It takes inspiration from 80s classics like Carrier Command and Midwinter giving it an interesting single player edge, but there's still much work to do.
Another Carrier Command inspired title is Carrier 2. The graphics looks very nice but the gameplay still needs working on, so it's at a similar stage of development to Thunder & Lightening. Still both projects have been in development for years so I am optimistic they will eventually turn into excellent FLOSS games.
Back to more contemporary planes, with glHorizon which is a freeware Windows only game. Focused on the F22, some of the visuals are spectacular. No "news" for over a year on this one but I have a gut feeling there'll be updates. The full 3D cockpit looks gorgeous. I hope the author releases the game under an open source license and then it can be embraced by the FLOSS game community. :-)
Then we come to the Combat Simulator Project. This really does look gorgeous. It has been in steady development for over 5 years and aims to provide cross-platform, high-fidelity, large-scale combat scenarios. It is probably the most ambitious of all the open source combat simulation games. The game is still at the demo stage but they are well on their way to acheiving their goals. Hopefully a new version (0.6 was released in April 2006 and is Windows-only) will be out soon. Like glHorizon, it sports 3D cockpits which look pretty cool.
FlightGear will most likely never support combat but is the leading open source flight simulator game so always worth a mention - I mean, you can fly combat planes even if you can't fight with them.
<update> Palomino is another less-combat oriented flight simulator, but looks great none-the-less. Given they showcase fighter jets, I'm hopeful combat will eventually get added to Palomino. <update/>
There are a few older titles that are no longer developed but once showed promise. Vertigo looks like some of the flight sims I used to play in the early 90s - nostalgic. ACM is another retro freeware flight combat game. The windows version is no longer available - I don't know why but that makes me smile. BFRIS is abandonware and I can only find the Windows version to download, but it has an interesting claim to fame:
First game to ship with Windows 95/98/NT *and* Linux binaries on the same CD out of the box. (from Moby Games)
The helicopter combat simulation scene is less healthy. There's Eagles that looks surprisingly good considering it's 10 years old - it looks like it uses a voxel based renderer, something I've not seen in a long time. Decopter also looks really promising but no updates since 2003 give little reason to hope. So it looks like you are stuck with the rather non-combat related Search & Rescue which is a bit more of a playable game than the other two but, as I say, not at all combat oriented.
If you are after a flight combat fix, look no further than GL-117. However there is plenty more to look forward to, so keep an eye on the scene, especially Thunder & Lightening and the Combat Simulator Project.