UltraVNC is a flavour of VNC — its an open source project in SourceForge. It provides all basic VNC features: connecting to remote machine, perform mouse/keyboard tasks on remote machine, transfer files, etc. It even allows connection via HTTP using a Java Viewer, possibly allowing it to cross firewalls.
Configuration is straightforward with a single password, but I’ve yet to see if it can integrate into the Windows Authentication.
So far UltraVNC has served me well and hasn’t given me serious problems. Server installs cleanly, and the client is light and doesn’t even need installation. A simple unzip and the viewer can be run.