Back in the ancient times of PC games, no one thought about shadows. The first attempts consisted of static black pixels, followed by small black circles underneath the characters. With time passing shadows got adjusted to be shaped like the person or object illuminated. By now they became dynamic, per-pixel-based and soft. Characters even cast shadows on their own bodies.

Everyone interested in the techniques of shadow making, might want to take a look at Wikipedia. There you are most likely to find explanations to keywords like Shadow Volumes.

Update: What's new?
To demonstrate the most important games in the history of shadow effects, we put together pictures from Pong on the PC (obviously without any shadows) up to Duke Nukem Forever. The second part adds current games and some more classics and features over 70 images.