The viewer has lots of goodies, such as file transfers (via copy/paste), desktop scaling, and of course, encryption when paired with the RealVNC Enterprise Server. It also has a 'listener' mode, so you can accept incoming viewing requests from the machines you support to traverse firewalls, etc.
[robg adds: To get the viewer, you'll need to download the ">Enterprise Edition demo, then run the installer. Inside the installer, you can do a custom install that installs only the viewer. This worked reasonably well for me on machines running Vine Server, though some features didn't work. I also had occasional issues getting mouse clicks to be recognized -- I suspect that I'd get better results with the RealVNC server, but haven't had a chance to test it yet.]

