The Student Room Group
I've used Inkscape in the past. It's free, but does take some getting used to. However, there are lots of very good tutorials on their site. There is a mac version available.

http://www.inkscape.org/
Reply 2
If you have lunux (which I guess you don't because you didn't say) there are loads of free software that does this, just type "sudo apt-get install _"

where _ is the name of the piece of software you chose when you searched google for "linux chemistry diagram", or go to System > Add or Remove programs and type chemistry - there are tens to choose from.

Anyway inkscape is the best if you want to draw it yourself (on any platform). You can also get SVG files off wikipedia and edit them to make them your own!
Reply 3
ACD Chemsketch: http://www.acdlabs.com/download/chemsketch/

You can use it to draw structures as well.

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