30 day price match guarantee
Did you know that Amazon.com has a 30 day price-match money-back guarantee? They don't match other retailers, but if you buy an item on Amazon and then notice within 30 days that the price of the has dropped, Amazon will refund you the difference in cost?
I didn't know about it until Matt Cutts linked me to Neil Gaiman who linked to a Slate article about Amazon's secret price guarantee.
Use a free service like this one or this one to automatically notify you if the price drops.
Not bad. The anecdote in the Slate article references a guy who saved $20 with this.