LogMeIn Ignition for iPhone
Various apps can control your PC from a distance, using VNC. For its ease-of-use, especially when reaching behind home routers, I like LogMeIn Ignition. This proprietary software securely--and effectively--lets me manage far-away PCs.
Note: This link takes you to PCWorld's iPhone AppGuide, where you can read a full review and download the latest version of the software.