The big boys of the free email world, namely Google, Yahoo and Hotmail have zero tolerance towards users who abuse their mail accounts and use it to send UCE
(unsolicited commercial email) to other email users. Here are a list of links which can be used to report such spam incidents so that their accounts can be permanently disabled by these providers.
Updated on Sep 1, 2012:
Feel free to share any other resources which can be of use to report spam to other such providers of free email services.