The best part is that it is drop-dead simple to use. Go to LogMeIn.com and get a free account. Once that is is setup, add a computer to list of computers you can control. If the computer you want to control is the one you are sitting at, the LogMeIn.com downloads a java application to your system. It usually takes less than a minute. If the system you want to control is else where, LogMeIn.com asks for the email of the user and sends them an invite. Once they accept, the software is automatically downloaded and installed on the remote system and then appears on your "My Computers" list on their Web site.
After that, it's a point and click to remote control a system. You'll have to know the account and password of the system you are logging into of course.
LogMeIn.com's remote control has some nice features. I particularly like the screen resizing. It scales the remote screen to fit in the browser window. Another completely cool feature is remote printing. When you logged into the remote system they can make the default printer on the remote system point to your local printer. Now you can run applications remotely and print just as if you were sitting there. Ditto for the clipboard. Also, the connection is always encrypted using SSL which makes it safe for all sorts of uses like medical and legal applications.
The basic desktop remote control package is free (amazing). For a measly $10 per month you can bump up to the IT Pro version that gives you file transfer and an great assortment of remote system management tools that allow you to interact with the system without interfering with the remote user should they be using the system.
I been using LogMeIn.com for 2 years now and have found it to be reliable and consistent. Highly recommended.