EasyShift and Gigwalk are two apps that let you sign up for small tasks around town and then get paid for completing them. These typically are micro jobs that you complete in under an hour for small amounts of money, perhaps $10 or less. However, Gigwalker says the average "active" worker on the service earns between $200 and $800 per month, so it can be worthwhile. And the freedom to sandwich these tasks into your schedule can be ideal for students or busy mothers.
Both EasyShift and Gigwalk work under iOS, and most people probably will run them on the iPhone, but they also work on the IPod Touch and the iPad.
An excellent review of both apps was written recently by Katherine Boehret, tech columnist for the Wall Street Journal. Here is her review of EasyShift and Gigwalk.