Minecraft wiki entry for Development_versions, associated text: Added option to enable single-player commands to create world screen.
The following commands are available: /gamemode, /toggledownfall, /time, /give, /xp, and /kill.
These commands are enabled in previously generated creative mode-worlds.
While using commands in singleplayer, the world around is paused, unlike in multiplayer.
Typing / then pressing Tab ↹ will show a list of available commands; pressing Tab ↹ while writing a command will tab-complete its arguments.
Each command is now its own class internally.