HPF pay you for doing the searches that you do anyway. It works by installing an extra search bar in your browser, every time you use this to search you get paid (no popups, spyware, extra ds). Average payment for each search is around 2p-3p (or $0.04 - $0.06 US).
Before you ask, you are paid a percentage of the advertising revenue that they generate from Yahoo so it is a pretty future proof plan, certainly as long as Yahoo are still around!
For more info see www.earnsomemoney.tk.
