There are an infinite number of scams. Ranging from stuffing envelopes at home to make $38383853939 a week to (oddly enough) becoming a billionaire by buying a $20 a year reseller account on Ebay. Depends on what their angle is, I guess.
the stuffing envelopes: the ad says you could make thousands doing it. what it doesn't say is that it requires you to pay a fee to "buy into the program" and that there is never a guaranteed minimum number of envelopes you'll need to stuff. also, they'll usually require you to purchase your own materials....
the "get rich quick by selling" scheme usually goes like this:
scum: "hey buddy, how'd you like to make thousands of dollars selling your friends long distance phone service"
unsuspecting victim: "hell yes! how do i do it????"
scum: "all you have to do is sell LD to X number of friends and you could make $1K. <insert distracting ramble here> once you sell it to them, all they have to do is sell it to X number of friends and you get paid. easy as that we'll even give you your own website to expand your market globally!"
i saw one where all you had to do was get 3 friends under you...well before you get the prize (a p4 2ghz laptop or something) all your friends have to get 3 under them seven levels down...the catch you have to buy some ebooks for $5...i calculated it up and the people before they gave you a laptop would make around $5000