Employee Stock Purchase Plan

Employee Stock Purchase Plan.

If your company has one, you should be in it for as much as they allow. An ESPP allows you to buy stock at a discount (usually 15%) off of the low over a certain period (usually 6 months). If you sell it immediately, it's almost impossible not to make money (*). And it's a lot more than 15% APR, since you contribute by paycheck (e.g. your last contribution of the period makes 15% in two weeks!).
(*) The only danger is if the company stock tanked badly in the day or two between the end of the ESPP period and you actually receiving the stock. Unlikely for most people.