Is there a playable postgame?
The game reaches a fairly definite conclusion before the credits roll, so if you choose to continue playing you'll be sent back before the point of no return identified by you
attacking the Cerberus headquarters and mounting a full-scale assault on the Reapers
This is basically kicking you back to an auto-save, as any experience you gained during the final few fights seems to be reverted. Since you're brought back before beginning the above-mentioned missions, you're free to play them over to explore the different endings.