Here is a list of a few great, casual and puzzle type games that I find are great time wasters and make you think. Also they do not need a very good Mac to run.
Crayon Physics
http://crayonphysics.com
Edge
http://twotribes.com/message/edge
Faster Than Light
http://www.ftlgame.com
Nightsky
http://nifflas.ni2.se/?page=NightSky
Samorost 1-3 and Botanicular
http://amanita-design.net
To The Moon (More on an experience than a game but it's a total tearjerker. A great time waster too)
http://freebirdgames.com/to_the_moon/
Crayon Physics
http://crayonphysics.com
Edge
http://twotribes.com/message/edge
Faster Than Light
http://www.ftlgame.com
Nightsky
http://nifflas.ni2.se/?page=NightSky
Samorost 1-3 and Botanicular
http://amanita-design.net
To The Moon (More on an experience than a game but it's a total tearjerker. A great time waster too)
http://freebirdgames.com/to_the_moon/