I made quadruped robot and stuck with kinematics part, problems with center of gravity (Atmega16, 12 servo - current accuracy 5 degrees). I can't get it nicely walking, maybe someone can offer any tutorials for it or ideas?
Put a gyro on it. Now you can constrain your body to always be parallel with the Earth (by sampling the gyro). Now simply move your legs in a repeat pattern to walk. This pattern can be generated using 3-D trig in a similar way as in the link ignoblegnome posted. Now when you get it walking ok on a straight surface you can start thinking of algorithms to deal with obstacles/non-level ground. The way to do this would be to add sensors on your feet that detect when they hit the ground. Again constrain the robot to always keep the body parallel with the Earth (using the gyro) and have the legs move until they feel that they have touched something.
Does this help?