I've just recently started multi-boxing on my mac, and yeah - clonekeys is about the only solution available. Doesn't have many features - there's no pass through lists, so everything you type in one shows up in all of them. I've inadvertently waved at people 3x simultaneously many times now.

I have a little scripting/programming experience, so I keep being tempted to open the source to that and see if I can add any features, but with a 17 month old and another on the way, that may not be feasible LOL