Mocha VNC Lite v 2.3
There are several VNC clients out there for the iPhone, but this app is the only free one. It lets you remotely control any VNC-based server (including Mac OS X with its built-in remote screen sharing) from your iPhone. Just enter the IP address and off you go.
There is also a full version for $6 that adds additional support for sending Ctrl-, Alt- and other special key combinations as well as right mouse button support, which might be of interest depending how much you live inside VNC sessions