Bitmixer is one of the most preferred bitcoin mixer enjoyed by many people.
At bitmixer,you have to pay a minimum fee of 0.5% and additionally transaction fee on every transaction.
You get very cheaper mixing than other mixers.
Bitmixer.io closed down their service a few months ago.

I think ChipMixer is pretty much the most preferred mixer right now.
*slightly biased*Bitmixer's post about them shutting down:
https://bitcointalk.org/index.php?topic=2042470