Circuit design is fine to do on paper for the simplest stuff but a CAD program is nice for everything else. I'd been using TinyCAD on my P3 400MHz, 128M desktop but am trying to move away from this glacially slow system, not to mention minimizing my exposure to Windows. :)
One good answer for Mac users is the free version of CadSoft Eagle running under the X11 windowing environment (follow the link to install X11). While it's not a seamless Aqua interface, it is actually a heck of a nice program once you get use to using it and it has an integrated PCB designer that works quite well. The user interface is a little bizzare, but with some practice I'm getting faster at drafting circuits. So far, the parts library seems to be pretty comprehensive, too.
If you want the app to show up in your Dock, you can write a shell script and use Platypus to create a wrapper application (may take several tries to figure it out) or try mine below which includes the shell script. Click on the link, save to disk, unzip, and copy the Eagle.app to /Applications. This app assumes the EAGLE-X11 folder is already installed in /Applications.