Impossible wheel

The impossible wheel, also known as the BC wheel (after the comic strip B.C.), represents the most basic type of unicycle, consisting of a single, spoked wheel with pegs extending from the axle. The rider mounts the wheel by jumping onto the pegs while in motion or by putting one foot onto one plate or peg and pushing along like a skateboard.
This device has no drivetrain, so when forward inertia runs out due to gravity and friction, the ride ends. Newer BC wheels use plates which hang below the axle instead of pegs in line with the axle. This makes them easier to ride since the center of gravity is lower. 
The unicycling community favors the term "BC wheel" because the device is not actually impossible to ride; proficiency can be gained with a few hours of practice. Tricks, such as hopping, drops, grinds, and spins, are possible.
Ultimate wheel.
An ultimate wheel is a wheel with two pedals directly connected – similar to a unicycle and impossible wheel. It has no seat or frame but offset and functional pedals. There are several different designs: the most popular involves a solid or semi-solid disk inside a regular small bicycle wheel rim with the pedals attached directly to the disk. A less common style uses metal cross braces instead of wood.
Some mounting and riding techniques include: