Chicken is a VNC client for Mac OS X. A VNC client allows remote access to another computer over the network. Chicken is based on Chicken of the VNC.
CoRD is a remote desktop client for Mac OS X ported from the UNIX program rdesktop. It is a Universal Binary, and allows you to connect to multiple servers concurrently.
(commercial) Desktop Transporter lets you control other Macs on your network, view their desktops, and use copy/paste to transfer text and graphics between the local and the remote machine.
(shareware) Hostal allows easy control and editing of the Host file on your computer. Hostal supports both host mapping and host blocking. Hostal supports Time to Live to prevent stale host mappings. For users on a network, Hostal detects an existing Hosts file and incorporates those settings as well.
Jem is an easy to use GUI front-end for the synergy client.
OpenSSH is a free version of the SSH suite of tools that are fully compatible with the three existing SSH protocols. Read more
(commercial) Remote Buddy gives you control over the rest of your computer via the remote control and also improves the available functionality in those apps that already supported the remote control by themselves.
Schnitz Remote Lite
Schnitz Remote Lite is a free version of Schnitz Remote that allows you to connect to a computer of your specification.
teleport lets you use a single mouse and keyboard to control several Macs. Simply reach an edge of your screen, and your mouse teleports to your nearby Mac, which also becomes controlled by your keyboard. The pasteboard can be synchronized, and you can even drag & drop files between your Macs.
Tiffany Screens is a presentation tool, able to capture the content of the presenter's screen and send it to multiple other computer screens at the same time. Any computer can seamlessly become the presenting computer, no matter if connected wirelessly or through an Ethernet cable.
TSclientX is a Windows Terminal Services client for Mac OS X. It appears to be a regular Mac OS application but is in fact an assembly of free software working in concert to provide similar (better?) functionality to the official Remote Desktop Connection from Microsoft. Additionally, there is a nice tidy VNC Viewer built right in as well.
Vine Server, previously released as OSXvnc, is a free, open-source VNC server.