Welcome to a space game special on Free Gamer!
At the heart of open source space game development is Vega Strike, a 3D space trading and combat game. With a dynamic universe and dynamic econmies, there is massive depth to the game. However the current stable version (0.4.3) is aging. While there is an update to the development version, it is not yet stable nor easy to try but does include some great new models and art on top of various gameplay improvements. Feature-wise it rivals commercial counterparts and the next releases should restore Vega Strikes position as one of the showcase open source games.
The most well-known mod is Privateer Remake, a modernization of Wing Commander: Privateer - an offshoot of the Wing Commander series. Privateer Remake was forked to form Privateer Gemini Gold, a version more true to the original Privateer. Both games breathe life back into a popular franchise, dragging it into 20th century gaming with vastly superior graphics to the original. There are more Wing Commander game remakes being developed out of the Vega Srike mod stable, springing from the meta-project Wing Commander Universe that spawned Privateer Remake.
No Gravity is a 3D arcade space shooter set in a fantastic universe made of 5 intergalatic worlds. The website is one of the best looking sites I've seen for an open source game. The game itself does not look as pretty as more recent titles like Vega Strike, but it looks fun nonetheless. It's gameplay reminded me of X-Wing vs TIE Figheter although it's been a long time since I played that so I could be mistaken.
Another space game whose progress has been steady is Oolite. It is basically Elite for the modern PC and was developed as a response to the withdrawl of Elite - A New Kind from the Internet (although it is still available from a few freeware sites). Oolite continues to exceed Elite in many ways and should delight fans of the original as well as attracting new players to the Elite brand.
Then there's FreeSpace 2. I was somehow under the impression that the whole game had been made Free Software. However, I could only find the FreeSpace 2 Source Code Project which did not contain much useful information, let alone a useful download. Increasingly convinced that the original FreeSpace 2 data was required and not Free, my interest quickly waned. If I'm wrong then could somebody please correct me.
Sadly there are those game development efforts that never quite make it. I came across Reaper whilst poking around the Internet. It's development ceased in 2002. Another space game whose candle went out a while ago is Andromeda 9, so it joins the echelons of once-promising projects.
Then there is Parsec. Once a darling of Linux gaming, Parsec suffered from being closed source. By the time the developers realised their mistake and open sourced Parsec, it was too late to interest a future maintainer.
Update: I have stumbled across Crimson Space that looks intriguing - you can enter a planet's atmosphere, it has several ships - although I'm uncertain of how complete it is and development ceased in 2002.
For people wanting an immediate fix, I would suggest No Gravity, Oolite, and Privateer Gemini Gold. No Gravity has the polish, Oolite has the depth, and Privateer the great 3D graphics. For people who are not afraid of rough edges, Vega Strike and it's burgeoning mod scene will push your 3D cards and engross you at the same time; Vega Strike is the open source future of this genre. One thing is certain, the open source 3D space game scene is very healthy.
Please comment if I missed an important open source 3D space game.