which uses identity tech to warn you when anyone tries to log in to your Steam account from a different computer) is now available for all users, according to the official website. Previously, it was only being tested by certain users (including Gabe Newell, who even posted his password online during GDC), but now anyone with a Steam account can give it a try.
To use the service, you just need to update Steam to the latest version, make sure you have a verified email associated with your account, then just restart Steam. Then, any time you log in from a new computer, Steam will send an email with a code that you'll need to enter to use the service. If you get an email for a computer you didn't log in from, someone else is trying to access your account. Pretty easy -- now no one will be able to play with your Torchlight characters but you.