0 A.D. is an awesome cross-platform game that is fun, has stunning graphics and is completely open source.
There’s even a PPA for Ubuntu.
It works wonderfully on both my laptops.

They are looking for a round of donations to pay for some more development work, and as of this moment they’re $634 USD short. I’ve just sent $50 their way.
If you’ve got a few bucks to spare, please send some money their way. Or maybe you want to get into some development work, they have detailed instructions on how to do just that!