Obviously you shouldn't use this to gain unlawful access to a computer. But here's how you can get onto a computer without the password legally.
But if you're a geek, however, you'll eventually end up in a situation where someone forgets their password, you acquire a machine with an operating system you cannot access, or similar situation. I can't tell you how many times I wondered "how do I get on my computer without the password?" or how many kids had parent blocks they had to bypass by going into the administrators account and turning it off. "hacking the administrators account" was a google trend search term in 2009.
See this tutorial for info on how to bypass the password on the three major operating systems:
Windows, Mac, and Linux.