How can I send a message to all players in-game?

In League of Legends how do you send a message to all, for example, to say 'Good Game' and the rest

2 Answers

Just type:
/all your message here or Shift + Enter to type your message on all chat

If you want more information on Hotkeys and other commands check:
LOL Hotkeys and other commands

While Enter brings up team chat, Shift+Enter brings up all chat.

Conveniently, this is the same way it was in Warcraft III, where the DotA / MOBA model first reached such widespread fame.