Steam's older games are about to get a pretty big overhaul. It's not one that many users will notice, but Valve believes it will lead to faster load times and an updated, more consistent file layout.
Counter-Strike: Source, Day of Defeat: Source, Half-Life 2: Deathmatch, and Team Fortress 2 will soon be converted to Valve's updated "SteamPipe" content delivery system. Generally speaking, you shouldn't have to do anything to get it to work. Steam will soon automatically transfer your games from the "older steamapps/[username]" section of your Steam folder to "steamapps/common."
Valve does warn that you will need enough hard drive space for about two copies of the game, to allow it to copy over successfully. The process will be a one time deal, and "will take a bit longer than the usual download." After the transfer is successful, you can expect Team Fortress 2 to take up about 1GB less space on your hard drive.
While most users just need to sit back and let Steam do the heavy lifting, modders and anyone currently running a mod have a bit of work to do. Custom files will need to be manually copied to the new location after the transfer has taken place. Older mods that used ZIP files will also need to be repackaged as VPK files.
Game server operators as well will need to switch to a new download tool, called Steamcmd.
The new system promises, "faster and smoother downloads, faster game boot times and map load times, easier distribution, installation, and management of mods, [and] smoother dedicated server distribution and update rollover."
You can read up on the entire process on the Steam FAQ if you want to know more. If you'd like an early look at the incoming update, you can join the Team Fortress 2 beta to see how it works.