In 2007, 2008, and 2010 We've used small cylinders of HDPE to tension our chain. We simply bolt it on a piece of angle with the hole off-center, rotate to proper tension, then tighten it down .
(2009 We didn't use chain we did direct gears instead)
Here are pictures of it's implementation on those 3 robots mentioned above:
2007:
http://www.vandenrobotics.com/plogge...n/dsc07244.JPG
2008:
http://www.vandenrobotics.com/plogge...n/dsc07275.JPG
2010:
http://www.vandenrobotics.com/plogge...n/sn850302.JPG