Unless the prize is large enough, most people are turned off to plain satoshi for captchas sites.

I like the concept of this faucet, even though they seem to have run out of funds:
http://bitcoin.lift-institute.com/lottery/
Every 30 minutes, users can draw either a variable amount of satoshi or a number of tickets to enter a raffle for the larger prizes.

This is also an interesting, though simple game, where the top 10 score earners win the prize: http://www.landofbitcoin.com/user/the-island (you have to be logged in to see it).

And if you really want a challenge, how about a browser-based mmo faucet. Something like http://browserquest.mozilla.org/.