For some people I am exactly the wrong person to ask about this. I am not artistically gifted; if anything, I’m one of those people who has trouble drawing a straight line even with a ruler. So what suits me here may not suit you, particularly if you’re artistically inclined. For me, doing drawing and diagrams on a computer is a godsend, because that’s the only way something even remotely reasonable can possibly be generated.
To wit: I don’t use Adobe Illustrator as anything other than a way to give a quick edit to something in a PDF file. Illustrator, for me, is like giving a huge load of fireworks to a 9-year-old. It might be fun and something pretty could happen, but mostly it’s just really dangerous for everybody. It’s like going after a fly with a bazooka, etc., you get the analogy.
For me, Illustrator is too general-purpose. Most of the drawing and diagrams that I do are flowchart-like system diagrams and stuff like room layouts. I don’t freehand draw, I put arrows in between labelled objects. My tool of choice for this kind of thing is OmniGraffle Pro, despite the goofy name—WTF is a graffle, anyway?
I think of OmniGraffle as a diagramming tool which also happens to support some more general-purpose drawing. While it has some foibles, it is generally an excellent piece of software which not just lets me do what I want, but helps me do what I want, because in this domain, assistance is much appreciated.
Years ago I used to use Inspiration as both my outliner and my diagramming program, but Inspiration, while it runs in OS X, really hasn’t kept up with the times.
The bad news is that OmniGraffle isn’t cheap. The Pro version retails for $200 ($120 academic), which is not trivial for software that isn’t, for me, an everyday piece of software. However, when I really need a flowchart or something similar, I really need one, and OmniGraffle is the way to go for me.
I’ve looked at other programs like LineForm and Intaglio and while they seem like nice drawing programs, they aren’t really diagramming tools, and I need a diagramming tool. LineForm is sold explicitly as an Illustrator competitor, and while certainly a lot less expensive than Illustrator, it’s $100. (Considering that academic bundles that include Illustrator can be had for only a few hundred, that’s not much of a savings.) Intaglio is somewhat less full-featured and slightly cheaper ($90). Intaglio does have one particularly cool feature for those of us who have been around for a while: it can open ClarisDraw files. I wasn’t a big user of ClarisDraw but that may be a big deal to the few folks out there who were. So, if you’re in the market for a moderately cheap but decent drawing (but not really diagramming) program, though, those seem like good things to look into.
I guess the other tool to discuss here is Zengobi’s Curio. Curio is a funky but likeable program but it’s very hard to describe. It’s sort a graphic information management tool, and it includes drawing tools and outlining and a whole bunch of other stuff all in one package. While I do occasionally use Curio, I don’t use it as my primary diagramming tool because it just isn’t as good at diagrams as OminGraffle. Curio’s name will come up again (and could have already as a presentation program, because it’ll do that, too) because it has features that touch on many other areas.