LogMeIn is a free service that allows you to remotely control Windows-based computers securely from any web-connected browser.
To use it, you simply need to create an account, then download and install the LogMeIn host software from the computer you will be controlling. That computer will then show up as available whenever you log into your LogMeIn account, and it is connected.
I like the fact that I can now quickly log in and fix problems for friends and family when they call, as long as I'm near and internet-connected computer.
Given the number of calls for help I receive (and I bet you do too), this has definitely saved me time.
There is also a paid version of LogMeIn with additional features like file copying and the ability to print to a remote printer, although I haven't as yet seen any need to upgrade.
Props to DownloadSquad