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Douglas_Knight comments on Actually existing prediction markets? - Less Wrong
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Truthcoin and its cousin Augur deserve a mention, even though neither is actually operational yet. (They're decentralized prediction markets on a blockchain.)
Idea Futures is still running (play money), but is functionally nearly dead and has very low liquidity. Once upon a time it was the best option for play money markets, and quite good.
Fairlay is a half-decent (though centralized) crypto market, though it's structured as a "betting market" and has no way to sell back predictions at a profit or a loss (you can place later predictions to hedge your risk equivalently, but you end up tying up a lot of money). Liquidity is bad.
BitBet runs some sort of weird time-weighted pari-mutuel system that I don't like, and has a lot of complaints about shady operations (eg very, very bad customer support that results in people losing money due to interface mistakes), but they often have actual liquidity.
As far as I can discern, the current state of things is abysmal, but I'm pretty optimistic about Truthcoin (less so about Augur in some ways, but optimistic there too).
Worth noting that PredictIt's odds should give you pause: there's money on the table betting "No" on all the presidential candidates, and I find it concerning that they can't interest anyone in arbitraging that away.
It is common across prediction markets that the fee structure makes it not worth it to push extreme events to further extremes. Thus unlikely candidates have too much mass and the total adds up to more than 1. But maybe Predictit is even worse for the reasons Anders gives.
You can build systems that preserve sum of probabilities = 1. They'll still see bias away from the extremes, because of fees and because of time value of money. But you can do a lot better than PredictIt. (One thing that helps on the fees side is to make fees go down for trades near the extremes; I argued for that in detail on Augur here.