Really want to recommend InTrade to LWers next election cycle. I put in $2000 and am up about $1200. Obviously if you follow politics intensively (or Nate Silver) you can make lots of smart bets against people betting on what they want or on the popular media narrative. But the markets were illiquid enough in this cycle that you can grind out more modest profits on arbitrage. For instance, even after it was clear Romney was the nominee, I could pick up 1 share Romney win + 1 share Obama win for about $8.20 and one of those bets was going to pay out $10.
If Nate Silver or the equivalent is making public predictions, it might be a lot harder to find people to take the other side of the bet.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.