How to play Minecraft offline?

The MCPatcher launcher has the capability to launch Minecraft, supposedly to be used to test whether your set of mods work with each other. In the 'Game' tab of the launcher, press "Test Minecraft" to run it in offline mode. Note that you will not have the same inventory as before, your username will be "Player", and you will have the default skin. You can only join offline mode servers that don't already have someone named "Player" logged in.

You can download the latest version of MCPatcher from the Minecraft Forums thread..

EDIT: After some testing by the asker, it seems this is not how MineCraft works any more with the new launcher.

If I understand things correctly, you have to have logged in with the specified user name at least once while online in order to play in "offline mode" with it.

You will also need to be online at least once in order to download the actual game (the launcher does not contain any game binaries, just the code needed to verify your login and start up the game).

If you copy the .minecraft folder from a computer which you have logged into MineCraft before, you might be able to just copy over the config files to the "offline" computer.