EA released the original C&C for free, and I think there are services that let emulate Westwood Online for online play, but if you are using any version of windows newer than 2000 (so, basically anyone on this forum and almost everyone else) than LAN play won't work.
Trash is free, you can play as the Humans. If you purchase the game, you get access to the Mutant side as well. I haven't played the game, though so this may not be 100% correct. I'm at work and cannot verify details for you. But feel free to check it out and let us know.