Here's a system.yaml that includes 3 bots I've been playing with, if you're bored with beating up Hard AI.
Zhukov AI: Bit of a turtle, but sends large attacks with massed infantry and arty/v2.
Rommel AI: Modified Hard AI, attacks often with small-sized squads, including arty/v2 mixed in with the tanks and infantry. Rommel doesn't like light vehicles, and should put up a bit more of a challenge to players that like to mass air.
epAI: Mostly ignore this. My little play thing.
For windows users: Go to c:\program files\OpenRA\mods\ra\rules. Back up the current system.yaml and drop this one in it's place.
Linux: /usr/share/openra/mods/ra/rules, same thing.
Mac: open up the OpenRA.app (it's actually a cleverly disguised folder) and drop it in the same place.
Link: http://dl.dropbox.com/u/1832211/system.yaml