Slush Pool a Czech based is the oldest mining pool that accounts for about 4% of the total hash power on the Bitcoin network. It was formerly known as bitcoin.cz and was made available on November 27, 2010 under the name Bitcoin pooled mining.
Earlier only the pool operators knew how much has power each miner was contributing but had no idea about what other miners were contributing. They simply had to trust the pool operators for that, but slush pool came out with a solution.
The solution as introduced by Slush Pool is straightforward. Rather than keeping the almost- valid blocks for themselves, Slush Pool will publish them for anyone to see.