+1 on ScreenConnect being awesome. Here are my top five:

• allows support to remote in to a client user's machine in under five seconds
• can configure users as support agents with limited rights such that they can only log in to their own computer. Handy for those who occasionally need to remote their computer, either from home or elsewhere.
• Android and iPhone/Pad apps.
• customizable to your company look/feel including colors & logos
• ad-hoc remote sessions with emailed invitations for systems that do not have a client installed

The version I use requires a small client (<2 MB) installed on the controlling machine, though all access is through a web browser.