These are single player games we find interesting but don't play as a group

Flash Games

Desktop Tower Defense
http://www.handdrawngames.com/DesktopTD/Game.asp
Build towers to stop your enemies from getting from one side of the map to ther other... involves resource management, maze construction, and knowing your enemies, several different types of structures and upgrade levels per structure.

TSG:Missions
http://www.casualcollective.com/#games/TSG_Missions
The whole name is "The Space Game : Missions" it's similar to Desktop Tower Defense to some extent... if you like one you will probably like the other.  This one makes power management and connections critical to maintain and not maze building.

GemCraft : Chapter 0
http://armorgames.com/play/3527/gemcraft-chapter-0
Another Desktop Tower defense style game.. this one uses more "resource planning" and less "maze building"

Magic Pen
http://www.bubblebox.com/play/puzzle/975.htm
You have a ball... and a set of Terrain with active gravity and physics... using your special magic pen and Hinges or pins can you build ramps etc to get the ball from where it starts to the flag?  It's kind of addictive but you can beat all the levels in one night.

Single Player Games

Dark Messiah of Might and Magic
http://store.steampowered.com/search/?term=Dark+Messiah+Might+and+Magic%E2%84%A2
It's availible on STEAM... the game is basically a single player RPG... however it plays almost like a first person shooter at times... and you definately use your environment in combat... if you DO NOT use the environment in combat odds are you will never complete the game.  By using the environment I mean kicking people off ledges, bashing them into spiked walls, smashing support stuctures to knock large objects on thier heads, Fire actually burns people and makes them scream and run around aimlessly.  Good times...