Here is probably the simplest robot leg that allows forward and backward and up and down movement. It only requires a toy geared motor and some other miscellaneous stuff to build. I didn't have to buy anything to build this project.
The problem with leg movement is that as the leg moves forward or backward it also needs to go up to prevent dragging it's foot on the floor. The wheel has all the correct motions built in and it is just a matter of attaching the leg to the wheel in such a way as to take advantage of that range of motion (using crank/slider mechanism).

Future modifications:
1. I plan to add a knee joint to the leg that will lift the foreleg or bottom of the leg up as the thigh moves forward or back thereby making it easier to step over obstacles..

2. It would be really cool to add a solenoid to lower and raise the pivot point of the leg. This would reduce and enlarge the leg stride on the fly making a way to increase and decrease the efficiency of the robot leg while it is moving.

3. It would also be interesting to put 6 of these legs together and see how a robot can move without using computer control to coordinate the legs movements.

4. Put a shoe on the leg - noticed it is slipping a bit on the carpet.

Step 1: Mount Motor on Tether

The motorized wheel comes from a toy construction truck. I mounted the motor on an acrylic ruler by drilling two holes through the ruler and into the motor gear box. Be careful that you do not drill into the gears. Then use self tapping or wood screws to attach the ruler, which serves as the tether, to the gear box. Note I hotglued and zip tied the motor wires to the gear box so that they don't get pulled out.

After that I drilled and bolted a spacer which is the plastic box (gotten from half of a power supply box) to the bottom of the ruler or tether.

Onto the spacer I bolted another short piece of acrylic ruler which will serve as the mount for the leg pivot point.

Step 2: Build Leg Pivot Point

The robot leg pivot point is just a u-shaped piece of metal (or could plastic or wood) that holds a bolt onto which the leg will be bolted.
The second picture shows that the leg is a slotted piece of ruler which will slide up and down on the pivot point or pivot bolt.

Step 3: Add Leg Drive Bolt to Wheel

Drill a hold in the wheel and bolt on a 1.5 or 2 inch bolt to which you will attach the leg. This is what drives the leg up and down and forward and back when the wheel turns. Note: the leg needs to be able to rotate on this bolt so do not make the nuts too tight.
I put the outside nut on loosely and then added a glob of hotglue so that the nut would not come off or tighten itself up.

Step 4: Build the Leg

The leg itself is about an 8 inch long piece of acrylic ruler.

Drill one hole towards the top end to mount to the motor wheel.

Cut a slot in the leg about the length of the wheel diameter. To do this I just drilled a bunch of consecutive holes, took an old soldering iron and melted the holes together until I got a rough slot. Frankly, I am amazed this thing works as well as it does with such a crude slot. It could be more efficient if I smoothed out the slot so that the pivot bolt would slide back and forth more easily.

Putting washers on both sides of the leg, mount it to the wheel and to the pivot point bolt.

Step 5: Build the Tether System

Since it is a single leg, you need something to support it which is an acrylic ruler with the motor attached at one end and a counter-balance at the other end.

The first picture shows a piece of jump rope handle that I cut and then put a bolt through to attach the ruler that supports the motor.
Attach this plastic bearing about the middle of the support tether ruler.

The second picture shows a piece of old metal mop handle that will sit inside some weights to hold it vertical.

The plastic bearing sits inside the piece of metal mop handle and allows the tether to rotate.

The third picture shows how it should all look put together.

Add some counterweight to the other end of the tether to take some weight off the leg and to make smoother rotation of the tether.

Step 6: Add a Support Leg (peg-leg)

Finally you need a support leg to hold up the motor and tether while the leg is in transition forward or backward.
For this I used a hollow piece of fishing pole cut a little shorter than the fully extended leg. I put some wire shrinkwrap on the bolt so that the support leg would fit tightly but so that I could take if off if I need to disassemble the unit.

And that is it, your robot leg is complete and ready to operate