Comment Teamviewer! (Score 0) 247
It is NOT open source, but is in free as in beer for your use.
It supports Windows, Linux, and OS X.
Teamviewer is very use to use; I worked in IT at a business with about 800-1000 desktops and laptops, and i never experienced the users having trouble with it. They just called support, and it would go "Please click the "Company-Name Support" icon on your dekstop" "OK" "Now tell me your ID" "xxx xxx xxx" "And your newly generated passkey?" "xxxx" and they would be good to go.
I installed it on my grandparents (85 and 82) laptops to ease the time i had to use on helping them, with the laptops. They never had any problems with it.