Ratio Stadium
Ratio Stadium is an additive arcade game in which the players have to find ratios that are equivalent to each other - and do it fast and accurately. Play against friends, against the computer or against other ratio addicts around the world. Addictive and great fun - three thumbs up!
http://arcademicskillbuilders.com/games/ratio-stadium/ratio-stadium.html

Ratio Blaster
Ratio Blaster is a one-person Space Invaders type game. How quickly can you blast the equivalent fractions out of the sky? Three thumbs up!
http://www.arcademicskillbuilders.com/games/ratio-blaster/ratio-blaster....

First choose the front and rear wheels of the bike, and then start pedalling. How far do you go?

http://illuminations.nctm.org/ActivityDetail.aspx?ID=178

Join together cogs of different sizes and investigate what happens when you turn them. From the NRich website.
http://nrich.maths.org/public/viewer.php?obj_id=4771&part=note