How about all cards that can help your opponent?
Vault, Bishop, Embassy, Soothsayer, Masquerade, Ambassador, Council Room, Governor, Pirate Ship... Minion, I guess? I've had so many lucky redraws with that card...
Based on http://wiki.dominionstrategy.com/index.php/Non-Attack_interaction, I'd go with:
Vault, Bishop, Embassy, Soothsayer, Council Room, Governor, Duchess, Envoy, Advisor, Contraband
I started putting together a "help your opponent" board for kingdom design contest, but never finished it. Soothsayer is an attack, and Envoy, Advisor and Contraband aren't really helping your opponent, they're just presenting your opponent with choices about what you do. I'd go with Tunnel and Market Square to enable Vault and Bishop more, and since there's draw and money, why not throw in some kind of action plays for good measure. Then you're building an engine to play even more things that help your opponent. Maybe...
Vault, Bishop, Council Room, Governor, Duchess, Embassy, Market Square, Tunnel, Throne Room?, Village?
I wonder about Procession instead of Throne Room maybe... there's MS and vanilla Village at $3, Procession and Bishop at $4, and assorted $5s to gain.