The SDK Skeleton implements Source, and nothing more. It is the ideal cruft-free base from which to build your own new game.
Extract the archive to a folder already containing the SDK's Source 2007 code (it won't overwrite anything). Open one of the .sln files with Visual Studio 2005/2008/2010 to get started. The player entity is configured for multiplayer but will work fine in singleplayer.
The compiled DLLs will appear in in \game\sdk_skeleton. Edit the projects' Custom Build Steps to redirect them to your mod's \bin folder. You will also need to configure debugging.
Also included are
CNetworkedPlayerMovement. These apply Valve's most current multiplayer player techniques to
CRagdoll, without any extra cruft.
They are not included in the projects by default. If you choose to use them
CBaseNetworkedPlayer (or a derived class) will need to be implemented by linking it to the "player" classname, and its member
m_PlayerAnimState will need to be defined with your preferred settings in shared code. Remember to update bots too, if you're using them.
Jumping will not work properly until you edit the
PLAYERANIMEVENT_JUMP case in
RestartGesture( GESTURE_SLOT_JUMP, ACT_MP_JUMP ). Depending on how your models are animated you may also want to edit
CMultiPlayerAnimState::HandleJumping(), particularly where it sets the
idealActivity to "jump".
There are at the time of writing errors in Valve's CPP files that you must fix manually: