Not the best source, but I recall Mythbusters testing some of these things years ago without any success. That site is particularly scammy ($500 to get started

Not the best source, but I recall Mythbusters testing some of these things years ago without any success. That site is particularly scammy ($500 to get started as a dealer! "I made over $21k in less than half a day!") but do you know of any that are actually verified to work?