Probably Club Live/Bing. I still have so much crap floating around (usually with the Club Live/Bing logo) from that including stuff I still use everyday like my keyboard, go to screwdriver, 360 controller, Zune... as well as a bunch of other stuff like a Waffle maker, an iron, duffel bags, t-shirts, binoculars, DVDs, webcam... not to mention like $1000 I made off selling crap like copies of Vista and Office and a dozen or so extra 360 controllers I had no use for, including a pink one that managed to catch the ebay market at the right time and rake in over the retail value. Granted, there have been some casualties. The refurbished 360 they sent me died although I did play it pretty hard. The awesome higher end bluetooth keyboard got fucked up after I tried to take it apart and clean spilled soda out of it. It got to the point I knew every UPS guy and I was shipping shit to family members.
Also, Camel cigarettes. They sent me an offer when they first started those flavored cigarettes where if I mailed them a few UPCs from any smokes they'd send me a free pack before they were released. I filled out the survey afterward and after that, I did a few half hour or so surveys from them (usually on computer, occsasionally phone) where they'd send me a check for $20 or $25. One month, they sent me a thank you letter with a coupon for $30 off a carton (when they were like $40.) The cashier when I used it checked it for a minute or so and then said she had to make sure it wasn't forged. After that, they invited me to test even more new flavored ones and sent me 2 packs each of 4 different flavors and then at the end, a carton of my either my favorite or the winner (can't remember) in a black wood box. The packs were all white and in small black letters said Style/Type/something A, B, etc.
And a sentimental favorite would also be The Honey Nut Cheerios one from around that time where you could win a $5 check. I was winning at least once a month if not twice playing completely legit with one account the whole time. I lost track of how much money I cashed after $50. The funny thing was was the checks were printed with bright orange and yellow paper with the logo and everything on, so every banker I got would ask to show them off and I became referred to as the Cheerios guy. I think most of my luck came to working 2-10. I'd play at like 3 in the morning a little before going to sleep.
Then there was a survey group I got into run by one of the big cable companies. It ran for around a year and it was easy amazon gift cards. I made around $500 for very little time and effort. Through them, I replaced practically every VHS tape I had with a DVD. (Except Star Wars, I have the black box set)
Oh and I'd like to thank the guy from the street sign only advertised yard sale that collected all those Mountain Dew caps needed for the limited edition original XBox. ...And then selling it to me for $20. 
