As promised, Valve has surprised us all with the unexpected bonus supplied along with the Source SDK this week. It's Half-Life 2 Deathmatch, and you can download it over Steam right now and start chucking things around with the gravity gun across a pair of maps based on sections of the single-player game.
The image you see here originally appeared on Valve's Steam news page yesterday, and soon after the SDK appeared on Steam along with the 32MB HL2 DM download.
Half-Life 2 Deathmatch is a separate entity to Half-Life 2, running from a different executable, and features its own built-in server browser. The two bundled maps are the prison-based dm_lockdown and the streets-of-City 17 style dm_overwatch.
You start out with the gravity gun, but there are also pistols, SMGs and grenades along with the game's various other weapons strewn around each map, and you can select from player models including resistance fighters and Combine troops.
In addition to Half-Life 2 Deathmatch, Valve is also running a Deathmatch Map Making Contest with a cash prize of $5,000 and two runner-up prizes of $3,000 each. You can find more details on that on the contest website.
So then. Half-Life 2 Deathmatch. You can literally throw the kitchen sink at one another. That's got to be worth a (free) punt, right?