Yes, you can register any names available for free. But the entitlement is temporary. When your sites built on co.cc have gained popularity or even got a solid pagerank, the company co.cc will soon ask your for a fee based on your site reputation. You will either have to pay for the fee(recurring…), or the name will be made available for others to register.

Seriously, I almost donate to get additional registrations. If you are serious about your sites, you'd better register a formal domain name like dot com.

Not ready to pay for one? Try co.nr instead. The company has better integrity. (at least I've never heard about bad experience with them)

P.S. Please note that the aforementioned policy is only confirmed for co.cc. Not certain with others.