we use Dameware NT Utilities at work. Absolutely brilliant product. It's relatively inexpensive ($285) and does way more than you can do with just a remote session. You can kill processes remotely, reboot, view installed software, start and stop services, ad nauseum. Oh, and if you forget to install the client on a machine, you don't need to, it will install itself remotely (provided you have admin access to the machine.)

The only caveat is it's primarily for use on machines on the network. For machines off the network I use Teamviewer (free for personal use) or tightVNC.

Nice thing about Teamviewer is that it requires no installation or holes in a firewall. You can talk someone through downloading it and running it. They give you their account # (auto assigned) and session password (also auto assigned) and you're in.