TeamViewer is an app that lets you connect to Macs and PCs via the Internet to share desktops, transfer files, walk through presentations, and control other computers remotely--all right from your iPhone. It's even free for noncommercial use, making it a great choice for your pro bono troubleshooting for friends and family, or just for accessing your own Mac from the road.
The interface is simple: you open TeamViewer and you're prompted to either create a session or wait for one. A unique ID and password are created for each session, and you need to have TeamViewer on both devices (if you're helping someone, they can just download and run the small "TeamViewer QuickSupport module," so they don't even have to install the full app). Once a session is running, you can see the other computer's desktop on your iPhone. Your level of control depends on the parameters you set for the session (Remote Support, Presentation, or File Transfer), and the interface and menus are straightforward, with intuitive touches like a big "X" that either user can click to end the session.
TeamViewer isn't perfect--for example, it often feels less like a iOS app and more like a ported PC program, which it is--but it's surprisingly stable, fast, and (perhaps most importantly) free for personal use. With extras like integrated chat and heavy-duty encryption, TeamViewer is definitely worth a download for anyone who needs this kind of functionality.