Help Wanted: A free program to draw circular plasmids: for Mac or PC

[Obabá]

Hello Everybody!
As you can see from the title of this post, I need a program
(preferencially free or shareware), that can make diagrams of
circular plasmids.
I would appreciate a lot if someone could point out a site in the
web where I could get such a program.
Thanks in advance!

Angelo Tome'
Center for Neurosciences of Coimbra
Portugal