Free .co.cc offers two free domains through their free domain service and up to 15000 domains if you are using the paid service.
Ease of use
The interface is simple and straight forward, the who is function is easy to use and does not feel like you are using a free service, one of the best features is that you can store your DNS settings in your account preferences and set them as the default for future domains.
There are no major restrictions on the domain usage and you can even park domains on the service if you would like to do so. Some webmasters have raised concerns about the content on domains since .co.cc operates out of Korea but I have not heard or seen any cases indicating that there is any reason to be concerned.
There are no hidden rules or costs with your 2 free domains and you are not expected to have reciprocal links or generate minimum amounts of traffic. Commercial services however are subject to a $3 fee and therefore it is only 100% for personal use.
The primary paid service is buying more domains and you can buy domains in increments of 100,230,360 up to and including 15000. Of course you are not required to use them all at the same time you just have to buy the capacity in bundles.
I find .co.cc very open and fair about the usage of the domains to the point that they even include guides for configuring the domain with blogger or windows live. Many free domain services will work with blogger or windows live but it rare to see it advertised in that way. I would probably be happier if they allowed 3 free domains but I guess expecting more than 2 for free may be a little greedy.