Balloon Fight is, in non-harmful terms, classic multiplayer mayhem. I had no idea it was so "very similar" to arcade game Joust before I played it on XBLA a couple of weeks ago. I guess the concept of flying balloons is more comprehensible than riding some kind of ostrich with lances and stuff. Anyhooo... Balloon Fight was ported to the NES in the mid-eighties. It's clean and simple, but most of all, it's still fun.
Available now thru Virtual Console. You can also play it in your browser, here.