CoinURL and Adcoin let you shorten links, like AdF.ly, but instead of earning money you earn Bitcoins. When you share those links and somebody clicks on them, they'll see for several seconds an advertisement. You'll receive a few satoshis for every one of those clicks. CoinURL is a great way to passively earn some BTC. Just post your links on facebook, twitter, forums, etc, sit back, and watch the BTC roll in.
