GameFly cuts 75% off a selection of Valve game downloads for Windows. (The discount is reflected on the product's page.) Even better, coupon code "MAR20OFF" knocks an extra 20% off, with prices starting at $1 after discount. Note that some of these titles require a Steam client to play. Deal ends March 30. A couple best bets, with prices after coupon:

• Portal 2 for PC or Mac for $3.99 (pictured, low by $13)
• Left 4 Dead 2 for PC or Mac for $3.99 (low by $16)