There are tons of mystery shopping companies out there. Avoid those that make you pay, and, if anyone sends you a cashiers check to cover your shop while you return the extra- run away!
Here are some reputable companies that I work for. As part of my mystery shopping contractor agreement with the companies, I can't divulge what specific businesses they shop, so I'll give general ideas.
1. Market Force - Fast food, office supply stores. Nationwide. I have done dozens of shops for this company, they pay regularly and have fair reimbursements for most shops.
2. Certified Field Associates - Movies, merchandising. Nationwide. A division of Market Force. I love the trailer check assignments, very easy and most of the time you can stay and watch the new film. Movie assignments are generally posted on Wednesday, and go fairly quickly so rarely are email notifications sent. I would not recommend the merchandising assignments as $8/hr is much too low when you add in drive and prep time.
3. A Closer Look - Restaurants, Hotels. Most states (not in Idaho, and I miss is soooo much!) Generous reimbursements. They rarely send email notifications, so check towards the end of the month for the next month's assignments.
4. GameFilm Consultants - Restaurants (Steakhouses! I'm going to one tonight, yum yum!), Oil Changes. Nationwide. Fair reimbursements. Good about sending out emails when new shops are posted.
5. Kern Scheduling - Varies. This is actually a scheduling company- they post jobs from a great variety of mystery shopping companies. Nationwide. Typically have jobs that are difficult to fill (aka not as fun/rewarding to do), but it is worth signing up for so you can find more companies that have jobs in your area.
As I said yesterday, I'm signed up for well over 30 companies. These are my top five, check back tomorrow for five more great companies to work for!