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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/
 
For laid-back time wasting but offering a chance to be creative, I like Sim City. I have been working on a representation of my home state and place I grew up, Pasadena Md.
 
For laid-back time wasting but offering a chance to be creative, I like Sim City. I have been working on a representation of my home state and place I grew up, Pasadena Md.

That's pretty cool. Do you have the version from the App Store? It's on sale for half off right now but it's not the online version.
 
CIV 5... it devours time like no other game

Desktop tower defense (on Kongregate.com), classic, but great on ancient machines or laptops... horrendously addictive if you haven't played it before... hint: don't look up winning strats online, try it yourself

Also in that same vein - Bloons Tower Defense 5
 
That's pretty cool. Do you have the version from the App Store? It's on sale for half off right now but it's not the online version.
Actually I run it off Steam on my PC. I tried playing on my Mac Pro but the graphics just weren't as good.
 
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