The changes include:

Steam
- Source MODs will now automatically appear in the games list
- Improved startup process for Steam
- Fixed crash when creating a backup that would span multiple DVDs
- Fixed a bug that would unnecessarily validate Steam caches after a cache corruption
- Fixed Condition Zero: Deleted Scenes and Codename Gordon failing to backup correctly
- Fixed a bug where the message "the game files will be deleted and re-acquired" would repeat
- Fixed offline mode failing after not connecting to Steam for more than a month

Half-Life 2
- Optimization to the Steam filesystem to reduce game launch and level transition time

Source SDK
- Simplified debug-launch process for MOD developers. MOD developers no longer need to place a config file in their directory
- Added a registry key for MOD developers to use to locate a user's Steam installation located at HKEY_CURRENT_USERSoftwareValveSteamSourceModInstallPath