Mechs are big robots and they make sounds like this and you can have great fun assembling them and then blowing stuff up!
Metallblech
Planned as a game for Syntensity, there are many models ready for usage in Metallblech, but (IIRC) not one line of code has been written yet and the project is desperate for programmers. ;)
Coldest
Coldest actually allows you to assemble the weapon systems of your mech. That's quite an important part for this genre: first, carefully assemble your mech, then brutally disassemble enemy mechs! A readme would be quite a good addition.
The command `svn co https://www.nemebean.com/svn/coldest/` will allow you to pull the latest code [EDIT: and for more instructions check out this wiki page.]
Coldest code is under GPL, art is under who knows what. (Actually I do know that both textures and sounds wouldn't qualify as free-as-in-freedom free. :) )
LinWarrior 3D
Since the last time Free Gamer reported about LinWarrior, the 2010_03_20 "Hadron Collider" version was released (download here). Besides the fact that textures and models have been licensed under "good" CC licenses, code tweaks and visual fixes, the MD5Mesh format is now supported.
Mouse control is definitely the one thing I would like to see implemented ASAP.
For some reason any video I recorded of LW3D would not show the dirty and brown ground texture, I'm sorry for the distorted view on the game caused by this!
Conclusion
I like mech games alot!