The iPhone is a great platform, but there are times when you require the functionality of a personal computer. When you need to reach out and touch your computer from far, far away, NTRconnect is your ticket.
There are two versions of the program, NTRconnect Free, which gives you remote access to two computers (Windows, Mac, or Linux), and NTRconnect Pro (starting at $6.95 per month), which allows you access to as many computers as you'd like, along with features like file transfer and local printing of remote files.
For my purposes, the free version worked very well, allowing me to reach both Windows 7 and Macintosh computers that were running various applications.
Loading up the required apps on each machine is simple, and the apps are lightweight, though I can imagine circumstances where you'd want to move to the pro version.
If you manage a large number of servers, then the pro version will be a requirement, as it will be if you want to leave computers turned off and have the iPhone switch them on when you interact with the system.