Grdc supports both VNC and RDP. Grdc can tunnel both connections over ssh for security. The ssh tunneling supports key based or password based authentication and allows the user to select a custom server and port (if they differ from the defaults).
Grdc's user interface is much simpler than that of Vinagre. It allows the user to create and save custom connections and organize them by groups. Its connection quality settings are, unlike many remote desktop programs, quite simple. The user simply chooses the quality and the number of colors. Grdc also has a panel applet for the gnome-panel that lets users connect to their saved connections through a simple menu.
Another interesting feature is the "VNC Incoming Connection" protocol. This protocol lets users create new VNC servers with custom ports and passwords on the fly. This is especially useful if a user wants to let a remote user temporarily control their desktop.
Grdc does not support everything. It does not support tabbed connections like Vinagre but, in my opinion, tabbed connections are overkill for most users. It also does not support browsing a network for VNC connections with avahi as Vinagre does. This feature would be useful for administrators but is somewhat useless for the average user because most users will access the computer from the internet, not the LAN, and thus avahi support would be pointless.
Overall, Grdc is good for remote, internet traversing connections because of its ssh support and is useful to the average user because of its simplicity. Vinagre is still a better VNC client for administrators that have to manage many computers over a LAN because, on a LAN, encryption is usually unnecessary while avahi support is helpful and, when managing multiple computers, tabs are very helpful.
For screenshots see the SourceForge Screenshot Page
As of the writing of this post, the version of Grdc in the ubuntu repositories is woefully out of date and does not support ssh tunneling. Therefore one should use the deb provided by the project maintainers on sourceforge. Simply download the grdc deb (link below) and install (usually just double click). The grdc-gnome package is the gnome-panel applet and the grdc package is the main program.
-- Edit: I have packaged a 64bit version. Download here.
-- Edit 2: The 64bit version of grdc-gnome (the panel applet) for karmic seems to work in jaunty. Download here.