ScreenLeap and QuickScreenShare.
Both user your browser and only require Java. There is no registration or software to install. Screen Leap seems a bit more professional looking, but both basically work the same. Here is a snapshop of ScreenLeap once you are ready to share.
Here are the steps for QuickScreenShare - but they are basically the same for ScreenLeap.
First whether you want to share your screen, or view their screen. Put in a name and click start.
Allow Java to Run by clicking Run this time.
Then Click on Run for the JavaApplet pop-up.
Then you are ready to share :-)
I'll be trying these for the next remote support request I have.