Minecraft Offline Files Installer allows you to play offline for free without having a Mojang account. Normally you would have to login to Minecraft for the first time before you can play offline.
To use offline mode for free (For 1.5.2 Users) Download and start the Minecraft 1.5.2 Launcher (Here: http://bit.ly/minecraft-152) Put in a Username (It doesn't have to be registered with Mojang) Leave the Password box blank Click Login, then click Play Offline (If this mod isn't installed, it'll say Not Downloaded) Note: You can only play offline for free with the old launcher (1.5.2 and below), This is because the launcher was updated with version 1.6.2. However, If you have bought Minecraft 1.6.2 and are recieving the 'Not Downloaded' error, this program will fix it.