Ok I’m in love.
Its like gotomypc – but its a) free for personal user and b) works with both Mac and Windows.
I just used it for the first time today fixing my mother in laws computer. For some weird reason I could never get ichat to do the screen sharing thing with her computer (though it works with others). Teamviewer connected first try and I swear it was quicker than ichat.
Give it a go – you wont be disappointed.