Mac OS 10.5 only: With VNC built right into Leopard, you can remote control your Mac from any other Mac via iChat or the Screen Sharing client—OR any PC using the right VNC client. Apple doesn't advertise this, but since Screen Sharing is just regular old VNC (albeit with a much more grokable name), our favourite Windows VNC client, TightVNC, works with it just dandy—with one small catch. You must set a VNC password in Leopard's System Preferences in order for the TightVNC client to connect to it. (Or else TightVNC will give you an error about the server not supporting the right security protocol.) To do so, in System Preferences, go to Sharing, and from Screen Sharing, click on the "Computer Options" button. Set your password there and TightVNC will be able to remote control your Mac given its IP address.
To secure your VNC connection, see more on how to remote control your computer over the internet with VNC and Hamachi VPN.