As the Chief Information Officer for my family and friends, I'm always looking for easy ways to get onto their computers to help them fix whatever is broken. I've been a HUGE fan of LogMeIn.com for years now but even getting that client software installed on their machines over the phone can be difficult sometimes :) To the rescue is the remote access site https://join.me/
This service is brilliant, and free. You tell the person that wants to share their screen to navigate to https://join.me/ and click on the "Share" button. They download a 1mb client and as soon as it runs it brings up a link that they can copy and email to you. Or they can just read the code to you and you punch it into the same join.me interface under the "Join" option. It's that simple. There is even chat and remote control in the free version.
My only gripe is that it does not work with Linux. I tried running it with wine and it runs but I cant see the Linux desktop. However, as long as you're not trying to share a Linux desktop it should work just fine to view either Windows or Mac computer. Bravo to the LogMeIn folks, this service is great!