Mocha VNC Lite is a free iPhone app. They also have an "full" version, but the additional features weren't compelling enough to convince me to pony up $6. These features are:

• another keyboard
• support for right mouse button
• macro support
• ctrl-alt-del key

Mocha VNC and VNC Lite are categorized under Business Apps. Mocha encourages you to try the lite version before buying the full version.