Walkers by Hiro Labo of Osaka Electro-Communication University's Department of Electro-Mechanical Engineering

First, here are Hiro Labo's more recent walkers, which utilize the pressure of sublimating dry ice to push LEGO's pneumatic pistons:

WOW fast, right?

The following implementation of Klann's linkage looks like it mimics his patented version pretty closely, and the liquid nitrogen could be replaced by dry ice with similar performance (which is limited by the max pressure limits of the LEGO gear, not the source of the pressure)

For more details on how LEGO's pneumatic parts were used, see the following video:

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Simplified Passive Walkers

Next, here are some more simplified walkers that use gravity as an external force to walk passively. 

Here's our favorite which utilizes a shaped foot.  Tad McGeer's development of passive walking with knees is described here.

Tchebyshev linkage:  

Notice how rubber feet were added to increase traction on inclines, or when pulling the robots.

Due to their small sizes the linkage dimensions of some of these walkers had to be modified somewhat to be built in LEGO, but they still walk fairly well.
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Ghassaei linkage:

​​TrotBot linkage:

​Strider linkage:

​Klann linkage: