I think this one is the best.  There are many variations in the offers that you receive and you can make some great money if your tweets are clicked often enough.  Try it out:  Sponsored Tweets

This site is fun and really easy to do.  You have lots of opportunities and the promotions vary.  Many times they let you do the first click while you are submitting, so you get guaranteed pay for that one.  They pay you weekly and if you just post daily and do that first click, you will get paid each week.  Try it out:  MyLikes

This is an interesting site.  You can sort of pick an choose your tweet ads and they offer you ads at certain times, like after 10 regular tweets.  They also sometimes pay on commission as well.  Do it:  Magpie

This is a great service with some interesting features.  Not as easy to use as some of the other sites, but after a little study, you will get the hang of it.  Possibly more profitable in the long run.  Check them out today:  RevTwt