Points For Surveys is the JetBlue version of the site, and when I did a half-assed test with that site, it seems like it's 4 surveys between those two sites. Opinion Outpost is the 'parent' of both those sites, and I'll sometimes get a survey there even after hitting the four per day on OMC. (You can get United Miles at Opinion Outpost, but it takes doing a fair number of surveys to get them to deposit in 300 miles chunks)
MyPoints has United Miles- again you're not getting the same number of miles per survey as with OMC, but it does add up, especially if you also do a little shopping or hotel booking through their portal.
E-rewards has United miles as one of their redemption options for surveys. Their downside is that, save for certain special circumstances, once you pick an airline or hotel program from their list, you're kind of locked into only redeeming with that program, and United might not be the best one to pick if you also collect avios or something.
e-miles is a smaller 'pay per clickthrough' program that's owned by e-Rewards with the same redemption restrictions. It's good for another 500-1000 miles a year (or a few more hotel points) with little effort.
If you collect US Airways or AA miles, Harris Surveys also has US Airways miles right now.