Free money from credit cards
I have already made a cool free $200 from opening credit cards, making one purchase, and then closing the card via suggestions from Jonathan at MyMoneyBlog. Now I've just applied for another one with a free $100 gift card bonus. Using the 0% APR and no balance transfer fees, he has borrowed over $20,000 and placed it in an interest bearing account, which will net him about $1000 in taxable interest. Nice going, dude.
Cool! I have an investment for you, send me $5 and in a few weeks you'll get $50g9v0osdjg dollars back if you mail these postcards the first 5 names on your street.
In all seriousness, these are ineffeciences in the market where a sharp person can make a bit of $.