It is really curious how the most simple and basic things often produce the best effects. A good example of this is Win Eyes, as it is a trick that is very simple but also very funny.
Win Eyes places a pair of huge eyes on the desktop that follow you, from one side to other as you move the mouse cursor. This produces a fun and curious effect. It is also possible to place it on the desktop where you like, and also alter the size of the eyes to the one you want.
Win Eyes is totally free and requires no installing.