On the day of release, a lot of people were asking which game they should buy off of steam to be able to play FF. You need
Source SDK Base to play. Below are the games/packs that you can buy. All prices are in USD and as of the last update on this post. Subject to change... of course.
UPDATE: Valve has a full list on the steam store found HERE
CHEAP:
Half-Life 2: Deathmatch for 4.99
You get Half-Life 2: Deathmatch and
Source SDK Base.
Half-Life 2: Episode One for $7.99
You get Half-Life 2: Episode One and
Source SDK Base.
Half-Life 2: Episode Two for $7.99
You get Half-Life 2: Episode Two and
Source SDK Base.
Day of Defeat: Source for $9.99
You get DoD:S and
Source SDK Base.
EXTRA CASH:
Counter-Strike: Source for $19.99
You get CS:S and
Source SDK Base.
Source Multiplayer Pack for $29.99
You get CS:S, DoD:S, Half-Life 2: Deathmatch, and
Source SDK Base.
The Orange Box for $29.99
You get Half-Life 2, Half-Life 2: Episode One, Half-Life 2: Episode Two, Portal, Team Fortress 2, and
Source SDK Base.
NOT-SO-BROKE:
Half-Life Complete for 39.99
You get Half-Life, Half-Life 2, Half-Life 2: Deathmatch, Half-Life 2: Episode One, Half-Life 2: Episode Two, Half-Life: Blue Shift, Half-Life: Opposing Force, Team Fortress Classic, and
Source SDK Base.
TOO MUCH MONEY:
Valve Complete Pack for $99.99