A Bitcoin Laundry or Mixing Service is a service that accepts BTC payments, and returns the same BTC amount, only from coins that are unassociated to the original BTC. It is a privacy service that works well if it has massive usage.
What are some good mixing services? Is there any data on how well they actually anonymize (e.g. what size are their "stashes" or user base (bigger stash = bigger anonymity)?