BTCGuild is a mining pool
which offers proportional based rewards, where your reward is equal to the block value, multiplied by your valid shares submitted during the round. The pool does not take a fee from each block solved, and calculates rewards to the full 8 decimal points supported by the Bitcoin protocol. The pool is run off the donations of users, and is offering premium services at different donation levels to encourage users to donate a small percentage of their rewards. Donations are taken from the individual user's reward when a block is calculated.
2.5% donators (calculated on a per block basis) are able to claim their rewards without waiting for 120 confirmations of the block. This also acts as invalid protection, similar to DeepBit, where a user can claim rewards even if the block is later invalidated/orphaned by the network.
BTCGuild first opened up on May 9th, 2011.