There are http://bettingblocks.com/ and http://block777.com/, in which you can gamble on which pool will find a particular block.

I have also seen a few sites that allow people to gamble on the block hash or the "nonce" of the block (eg. Is the last number in the hash odd or even? Is the last two digit of the nonce <50? etc), but those sites didn't seem to get much attention and soon disappeared.