http://store.steampowered.com/sub/29197/
"Steam offers downloads of the Valve Complete Pack for Windows and Mac for $24.99. That's the lowest price we could find by $75, although we saw it for Windows for $5 less in December. It includes 24 titles, such as Portal 2, Counter-Strike Source, Half-Life 2, Left 4 Dead 2, and more. Select titles are also available for Linux. Deal ends October 1.Don't need them all? Steam also offers select Valve games for 75% off, including Portal 2 and Half-Life 2. "