I've got a few games that have multiplayer but are also good in single player

Half-life 2
Terraria
Any Elder Scrolls
Fallout
Borderlands 1 or 2
Halo series is good
NOT CALL OF DUTY
Minecraft (for some people. I personally hate the newer versions. I suggest 1.0.1_47 Alpha)
Just Cause 2 (one of the best single player game of all time)
GTA
Dishonored
Warframe open BETA (single or multiplayer)
Assassins Creed series
Batman: Arkham City/Asylum

If you need any more, I can probably get some