Remote RDP Light is a remote desktop client app similar to Remote Desktop for Windows that allows you to connect to most Windows computers, see the desktop, and control it as well as you can from a small touchscreen device. Surprisingly, that control feels pretty natural except for where precision mousework is required.
Unlike the previously-reviewed Splashtop Remote, this is free and requires no additional software installed on the Windows machine as Remote Desktop has been included since Windows XP.
While the Lite version is lacking some of the features the pay-for Standard and Enterprise versions have (such as audio, SSL, NLA, file transfer, clipboard redirection, unlimited sessions/connections, and more), I've found that for quickly fixing a simple problem or checking a document on my PC, this application is great. I had to use it a couple of time over Christmas vacation to diagnose problems at work 2,300 miles away.
One point to note though is if you're on the phone talking to the person and attempting to work on their computer, you'll need to be connected via 4G/WiMax or WiFi, as Sprint doesn't do voice and 3G data at the same time.
Links: Market (EVO) | Market (web)