Useful Commands
Changing the game mode
/gamemode creative- Changes game mode to Creative (unlimited inventory and flying).
/gamemode survival- Changes game mode to survival.
Environment
These are very useful if you're taking screenshots for a Minecraft tutorial website!
/weather clear- Stops it from raining.
/toggledownfall- Switches rain or snow on and off.
/time set day- Changes the time to sunrise.
/time set night- Changes the time to sunset.
Naughty cheats
/xp 30L playername- Give yourself 30 experience levels for enchanting (you can of course change the number, and instead of "playername" you should type your own name).
More commands
Type /help for help, or visit the Minecraft Wiki for more commands.