0 A.D. is a free game made by a community of volunteers and is an Age of the Empires style RTS. The game is only in alpha but they have a pretty solid build available for download on their site. The project still has a long way to go but amazingly you can already play it on Windows, Mac OS and Linux. I've tested it on Windows 7 and Mac OS 10.7.5, here's what I found.
Both versions seemed to run equally well, however, the game has yet to be optimised so when a lot of stuff starts to happen, the game starts to chug. The thing that strikes you at first is how beautiful the game is. The landscapes are incredible and most of the unit models are pretty good too, they just requiring some touching up here and there. Still it's pretty good for a community run project.
There are a lot of little bugs and features missing at the moment. One of the most annoying things is that workers will get stuck on each other and do this silly little dance back and forth, unable to pass one another. Like in Age of Empires you can queue up units to come out in one big go. You shift click on the units you want and that will increase the number by five, you can keep doing this until you let go of shift and it will plop out 5, 10, 15 etc. The downside to this though, is that there are no warnings for having reached your population cap and this technique can really chew through your allowance. There are also no battle notifications as of yet either so it can be a bit confusing when you come under attack.
The gameplay is a lot like Age of Empires but there are a few differences. One thing I found whilst learning about these differences, is that even set to the easiest difficulty the AI is quite difficult. They always manage to find the one weak spot in my defences and raid my base. The first time I won a game against the AI, I found the combat to be a little lacklustre. Skirmishers will run off in tangents firing at foes who get too close and because the game chugs when there is so much on screen it makes it very hard to manage.
The game clearly needs a lot of work but still, I am quite impressed with what they've done. It's the kind of project I will check back on from time to time to see how they are progressing. They are also always keen to have people work on the project so there is always that avenue to pursue, if you're interested. It's well worth checking out and I can't wait to see how it is when they enter the beta phase.