My previous computer came with a program called
OmniGraffle, and I have found it to be great. If was not pre-instaled on my new MacBook Pro, though. It's not free, unless it happens to be sitting on your computer already and you're not aware of it.
There's a quite good, but complex, vector drawing program like Adobe Illustrator called
Inkscape... the downside is that it isn't a native Mac app. It's a Unix app and requires X11 (available on your Mac OS X install disk as an optional install). There's also a similar program (also requiring X11) that is more like Adobe Photoshop called
Gimp.
Hope one of these can help you!