A replacement wheel (about 70mm in diameter) for the ubiquitous yellow cheap robotic gear motor / wheel combo.
This was printed in TPU, a flexible filament similar to semi-flex. The deflection of this wheel under load is very similar to the wheel it replaces. It could be printed in a hard filament like PLA but it would have much less grip.
There are customizers on Thingiverse that allow you to set custom dimensions for a wheel like this but none of them produce a wheel with tread.
A brief video of this wheel in action:
For more projects see: http://www.robot50.net
For best results print in a flexible filament like TPU.