Gives bitcoins out for free to each visitor. IP addresses that receive bitcoins are recorded, so that bitcoins are given only once to each address. It is operated by Gavin Andresen.
The amount of bitcoins given per visitor depends on the amount of bitcoins in the faucet account. Currently, when there are less than 500 bitcoins in the faucet, it gives out 0.05 bitcoins, or 0.50 bitcoins otherwise.
People can donate to the faucet account. When the bitcoin currency was new, this account could be easily filled, because the price of bitcoins was very low. It used to be the norm that 5 bitcoins were given to each visitor, but now only 0.05 BTC are given.
There is a separate TEST Bitcoin faucet that gives test bitcoins for people who want to do tests on the Testnet.