So, the main minecraft auth/registration server got rushed in a big way. And shit's broked.
The good news is that because of this,
"Until this all gets sorted out, I'm officially calling a "free-to-play weekend", or however long this will take to fix. You will need to have a paid account to play it later on when this gets fixed though."
So go here, download the client, and check out the multiplayer survival mode while you can without paying $10 (its worth it)
http://www.minecraft.net/
e: It asks for a username. Just put in whatever you want. I'm still using my normal name, but isauhoisadgfhosuzydgfiosuarht works too.