You can play all the games on Steam without any connection to the internet - you don't even need to do anything special.
Yes, you do need to do several things special. That is how Valve prevents people from sharing their Steam installations/files to have working copies of games. You may be too stupid to see the restrictions of offline mode, but they are there, and you will run into them sooner or later especially if your Steam account disappears.
Reformat your PC, reinstall Windows, and then try to run your games without
first authenticating with Steam and see how far you get. Offline mode relies on a stored hash, and that hash varies from PC to PC and Windows installation.
If Steam goes down permanently you can simply use the backup feature to keep them safe.
Steam's backups are useless without first authenticating and building the hash to enable offline mode on the PC on which you wish to play them. Therefore if Steam disappeared or you lost your account those backups would be worthless.This comment was edited on Jan 19, 2009, 20:40.