In my region there is a saying: "Not even the dog wags its tail for free".
Some pools charge more than 5%, some are free. Most pools publish their reward: https://en.bitcoin.it/wiki/Comparison_of_mining_pools
Some pools (i.e.: eligius, ghash, p2pool, itzod) charge 0% to users and even share the transaction fees.

Pools are expensive services to maintain, where is the deal for 0% pools?

Information gathering like Google? Fees hidden in the rawarding calculation?
Ghash could have a "production control strategy", but the others?

Who or what can stop a pool operator from adding a line of code in the users rewarding script, to multiply his own miner shares before the final calculation?