http://unknownworlds.com/ is a great example. When it was still just a halflife mod, and the creator wanted to work on it full time while bootstrapping the company, he asked for donations to get a special icon in game and playtesting access. There was an incredible outpouring of support and it kept them in the green.
Now, to fund the stand-alone sequel, they are taking preorders far in advance to help the development, with an option to pay $20 extra for the game to get special black armor in-game and their eternal gratitude. Almost all of the preorders opted to pay extra, mostly out of thanks for the years they spent playing the first game for free.