0ad is my favorite with my son. We can play as a team against the AI. It is a blast. I wish online was a better experience - right now, you sit around being shunned unless you have a high rating (you also can't get a rating since the highly rated players just game the settings and map).
Hmm, I heard this often about long-ran games that don't have much of a player turnaround...
You might be interested by another open-source RTS : Zero-K/Spring, where moderators take care to nurture a good experience for everyone, for instance :
