Inkscape is a free and open source vector drawing app for Windows, MacOS & Linux, available for download free under the GPLv3+ license. The tools aims to support illustrators, designers, web designers or anyone wanting to create vector graphics.
I would 100% recommend Inkscape to anyone starting out with graphic design or illustration, as it isn't that difficult to get the hang of on the first day of using it. I am not recommending Inkscape for beginners alone, even experts in the field can use Inkscape professionally as it cuts of the expense of needing to buy a paid graphic design software which provides almost the same features. Inkscape is also great for beginners because it can be treated like a stepping stone to move onto a paid graphic designing software like Adobe Illustrator, so Inkscape can be used to test the waters before making the leap. Inkscape and Illustrator are almost the same so there wont be any difficulty in switching later on.
Omnigraffle is great for documentation, mapping, flowcharting, and other technical diagramming scenarios. It's simple enough to bang out a quick illustration and powerful enough to build complex blueprints for complicated technical systems. If you need cross-platform compatibility, though, you're probably better off looking elsewhere. If you want complex integration with data sources (ala Visio's SQL Server integration for shape metadata), OmniGraffle also falls short — but those scenarios are few and far between in my experience.
OmniGraffle is fairly simple to use, but the one thing I think it does best is working with curved lines, particularly if you are using some of the available arc templates. Drop an arc onto your page, then tell it the dimensions it needs to be, and viola! Done. Manipulating the arcs is as simple as clicking and dragging offset points.
OmniGraffle has also done an excellent job in stirring up the creative minds of many people who create templates and tools to work with OmniGraffle (not that Microsoft hasn't done so either), and managed to get the bulk of those into well organized repositories.
What it all boils down to for me is: it just works. One doesn't need to have a computer science degree to work with it either. It is as simple or complex as you want it to be.
I'd love to be able to keep more than one of the different tool tabs open at a time.
The stencils are amazing. Would be great if a whole lot more of the free ones came standard as opposed to having to download them from Graffletopia or other sites.
Inkscape is the standard if you want a free product that can produce high quality vector based graphics, there may be other products that can do it better but they are very very expensive. The experience you will get from expensive commercial products will be better but if you are needing to produce vector graphics now and again, or the cost of a product to use is a big factor - Inkscape is the best choice.
While these other tools are great for what they are, OmniGraffle’s solid focus on and support for diagramming makes it our tool of choice for communicating workflows and concept relationships, creating documentation, and creating other diagrams. Its libraries allow us to create designs quickly, and its ease of use enables us to use the tool widely across the company without much time or effort spent on onboarding.
Omnigraffle isn't an expensive software tool, so there isn't really any negative from the perspective of raw cost. Thinking in terms of time spent using it on a project - what you create in omnigraffle will inevitably lead to a dead end. It's useful only as documentation. There are other tools like Sketch that integrate into prototyping software and can create useable visual assets for applications in addition to being able to create wireframes.
