Some games on Steam do.
These games include:
Day of Defeat: Source
Left 4 Dead 2
Team Fortress 2
Half-Life 2
Portal
Half-Life 2: Episode 2
Counter-Strike Source
Alien Swarm ( Free on Steam)
Half-Life 2:Episode 1
Left 4 Dead
Half-Life 2: Deathmatch
To download Source SDK, go into to steam > tools > Source SDK. You must have a source game to have SDK availible.
You can buy the game off of Steam. You require a Source SDK game to run Garry's Mod, but Steam sells these games.
Well, if you bought the PC version though Steam, you can download Source SDK (free) though Steam, under Library, Tools, then double-click Source SDK to start the download. If you need any help using it: http://developer.valvesoftware.com/wiki/Main_Page
diagrm of sdk
You should really be posting this on the Steam Community Forums, you would get a much better response there. :)
You can make a CS:S map by using Souce SDK's Hammer World Editor if you have a steam account and CS:S you already have SDK, to load it to your PC just go to your steam library, then to the tools section and under tools should be Source SDK once you download it (it shouldn't take as long a CS:S to download) go the the Hammer Editor icon and then design your map. If you need mapping help you can go to Interlopers.net or developer.valvesoftware.com, if you own HL or team fortress you can also make maps for those as well. Hope this helps :)
Wii Opera SDK was created in 2007.
not yet
Yes, Source SDK Comes with all the source games
You cannot do this in gmod. You need to use a program called Source SDK (software development kit.) you can find this program on the internet or on steam. (note: this program is extremely complex.)
If you have HL2 on PC just go to your steam page and go to library and click on the tools bar and you should see a icon that says Source SDK. SDK is free and able to be downloaded if you own HL2, Ep.2, Ep.1, TF2 or portal. once you download it, you can make, play, and share you levels.
You can make one yourself with po_edit_03 or Hammer(Available in steam apps under Source SDK.) Or you can visit www.thinkingwithportals.com and browse their availabe maps. To add them, put the zip file in steam/steamapps/(your account name)/portal/portal/maps and extract. Then enable the developer console in the advanced keyboard settings, and type in the developer console "map (map name here, make sure it's exact.)" And voila.