There are a variety of companies offering mystery shopping jobs on the Internet. I would be cautions because some of these firms are phony and will not pay you. There are some reputable firms out there such as: intelli-shop, epms (apartment shops), and iccds. You can also visit the mystery shopping provider's association at http://www.mysteryshop.org to learn more about what to watch out for. Typically, on restaurant shops the shopping company will require you to pay for the meal up front and you will get reimbursed once you submit the report to the company. There are also restrictions on what restaurants offer the shops and the locations. Keep in mind also that it can sometimes get competitive depending on the geography and where you live. You may get many people trying to apply for the same shop.
References :
http://www.mysteryshop.org, personal experience.

I have spent the last 4 years working on mystery shopping, inspection, and audit projects. Everything should be free to join, but it takes time to wade through all the Companies to register etc. That is why I decided literally today to set up a Yahoo group with free information. I also have a web site with more detailed free information if you want to visit it. There are people out there who will try to get you to pay for this information. I NEVER do. I may earn from the Companies I refer you too or from sponsors you CHOOSE to visit, but never FROM you. My first post will be tomorrow and it is open to join. Or check out the website.
References :
The Group is inspectioninsider and my website is http://www.ewanid.com