I've used service intelligence (secretshopnet.com). It's free and features direct deposit. There is usually a month delay until your money is deposited and you have to become certified for a store before you are able to secret shop there. Some examples of some places that have been available are: BestBuy, Walgreens, McDonald's, Home Depot, Church's Chicken, Quizno's, and others. One note of caution though... If you do not log on to your account for 90 days, your account will be disabled and the only way to shop again is the start the process from scratch --- including all of your certifications.
If you have a Captain D's near you, you can apply on their website. The reports are very easy to fill out and not that incredibily detailed (unlike service intelligence's). Also their checks arrive pretty fast. The most they will reimburse you for is $7.50 (last time I checked). But this is plenty if you are single or a small appetite couple. And unlike most other secret shopping opportunties, you can pick whatever you want to eat and as many different things as you want. Recommend highly!