Note: This does not work on OS X Mountain Lion (10.8.2). It has apparently been fixed in newer versions.
On Mac (machine to be controlled)
Open System Preferences (it's in the Apple menu)
Open the Sharing panel
Check the box Apple Remote Desktop
Click Access Privileges
Check the box VNC viewers may control screen with password
enter a password of less than 8 characters
Click OK
On remote system (machine to control)
Launch Remmina Remote Desktop Client
Select Connection > New from menu
Select VNC - Virtual Network Computing as Protocol
Fill Server field with either IP address or hostname of the Mac
Optionally fill both User name and Password to make Remmina remember Your credentials
IMPORTANT: Set Color depth to High color (15 bit) or better, otherwise connection will fail
Click Connect (or Save to reuse it later)
The instructions above are for a Linux machine with Remmina installed, but you can do the same from a tightVNC command line (cli or GUI, Windows or Unix). You can also access using other tools like krdc.