You could buy a keylogger

(...although I'm not sure why you would.)

I've mentioned keylogging before, and how a malicious Mr. Stranger could trick you into installing some keylogging software on your computer, which records everything you type (including your passwords and credit card numbers!) and sends that info to Mr. Stranger.

You can install software-based keyloggers to monitor use of your computer. There are also hardware-based keyloggers - you can plug your keyboard into this small keylogging device and then plug that device into your computer. Anything that is typed on your attached keyboard is recorded by the device. You don't need to get onto the computer to install the software. And because there is no software installed, anti-spyware programs won't catch it, and the person using the computer won't notice it (unless they examine the back of the computer and find the device.)

I'm not sure why you would engage in such spying activities - it's an invasion of privacy! (Although I guess you could be a concerned parent, or a school librarian monitoring your computer lab, or a friend that needs to find out your friend's password so you can strategically plan the best surprise party ever, or just a mischievous sibling spying on your little brother or sister.)

Well, I was looking through the online store at thinkgeek.com the other day to get ideas for gifts, and I ran across a hardware-based keylogger that you can buy. And if you search for "keylogger" on amazon.com, you'll find a number of different models, some even more sophisticated/covert. I bet you could buy a keyboard with a secret built-in keylogger as well.

I wonder how secure public computers in the library, hotel, school, or internet cafe are, and how hard it would be for a regular person to install such a device on these computers. (Don't try it.)