All retail Valve games and a number of third-party games can be registered to your Steam account. All of the following games can be registered through Steam:
Opposing Force CD-keys cannot be registered through Steam. Opposing Force was included in a retail Half-Life package. The Half-Life CD-key included in the package will unlock all of the games in that package, including Opposing Force.
Note: Only the retail games listed above can be registered through Steam. No other retail games can be registered and downloaded using Steam.
Tags: Digital Distribution