Left 4 Dead 2 is free right now on steam
store.steampowered.com
store.steampowered.com
edit: I'm not implying that Steam is being attacked, here I intend to say that the DDOS is organically driven by high demand for the content.
It's more likely that their servers are overloaded from the number of users trying to grab L4D2 along with the other winter sale items. Steam store has been iffy the past couple of days already.
(But even the Dota matchmaking is down right now)
edit: Perhaps I should have said "organically DDOSed" or something to that effect to better signal that I was using the term in a casual manner somewhat divorced from its malicious origin.
I'm putting this one over the Orange box.
Valve had basically offered it for $5 every Steam sale for the past few years so this is a mercy gift. (as with Portal last year)
So unless Valve intends on releasing L4D3 any time soon, I think it would be great if they put more work into L4D2.
In addition, keep in mind this is the game they chose to demo the feasibility of gaming on Linux.
Even if they don't it's very easy to get custom campaigns in the game yourself.
But there can definitely be some work done on netcode and various gameplay glitchy issues.
They probably have a growth curve that starts initially as a pay game, then on taper, open it up, then add in items/content and the markets. Very smart and they have done them one at a time. The markets and goods actually make the game fun to go back to and don't bend game balancing too much, but add a layer of fun. They are pretty much the model for good in-app purchases on desktop.