TeamViewer is a tool for viewing and controlling remote desktops.
You need to install it on two different computers, either Mac or PC, then enter an ID and password to open two ends of a secure tunnel.
At the moment TeamViewer for Mac is still in beta but it worked well for us. The other operator can choose to give you just a view of their desktop, or full control over it.
If the other person is inside the same LAN as you but in a different part of the building, you can connect using their IP address as well as their TeamViewer ID.
Control is enabled through a single window. It's a responsive interaction that works as advertised.
We connected to Macs inside our LAN and outside our LAN using TeamViewer's ID system and made it through firewalls without a hiccup. You get pull-down menus with options of what to do with the connection, including viewing the whole desktop or just the one window.
Our only problem was trying to share files through TeamViewer, which is an advertised feature for Macs that we couldn't locate in the menu system or user guide.
We put that down to the beta status of the release. In the meantime we just emailed our files across.
Free and easy
Other apps are out there that do the same task but with less features or different delivery methods.
LogMeIn is a free, web-based access service that recently won a MacFormat Choice award. iChat can control a remote Mac, as can Apple's paid-for Back to My Mac service for .Mac subscribers.
Against these, TeamViewer does very well for itself because it's free and cross-platform, and will probably be the leading tool for remote access when released.
We stress this in only in beta, but it worked fine for us and is easy to recommend.