One of the games that has really impressed me in recent years for its originality and simplicity is World of Goo. It's a physics puzzle game where you have to attach goo balls to make shapes, but bear in mind things like gravity, wind etc.
Normally it's $20, but until Sunday, it's pay what you like.
Runs on Mac, PC and Linux. Get it from 2d boy website.
Strewth Sheila, 'Straya has fake charities too!
7 hours ago