Although it’s not officially but Oracle Database 12c is available on edelivery.oracle.com (only for Linux 64-bit).
If you have valid credential and wants to test this new flagship database, download it from edelivery.
The documentation is still in beta. 😦
Update (after 2 hrs):
The docs available at Oracle Database 12c Documentation page.
Software available for Linux x86-64, Solaris Sparc64, Solaris (x86-64) on OTN as well as edelivery.
Happy testing… 🙂