Microsoft Research have a project which shows pictures of, say, animals and asks the user to identify which one is the dog and so on. Computers are very bad at this while humans are not:
http://research.microsoft.com/en-us/...ojects/asirra/
---------- Post added at 16:16 ---------- Previous post was at 16:13 ----------
Otherwise I would go for reCaptcha. It's not that cumbersome imo.