Several of us on the developerWorks development teams have been refreshing our systems, using Linux a lot more, having to go through our backups and application lists to get new systems set up and running.
I have been a CorelDRAW user for some time. Was never very good at it--designers really dig deep and master this vector drawing product--but I was competent enough to appreciate its power and, frankly, the reason for its expensive price tag. Since I'm not doing much authoring these days I'm not licensed to use Corel any more at IBM, which is a bummer, and so I went looking around for drawing tool substitutes, wondering especially if I could press the open source GIMP paint tool, which is perhaps best supported on Linux, into service as a draw tool and not just a photo and paint tool. People do it. But that wasn't feeling right and in the same places where I was seeing discussion of drawing beziers &c with GIMP, I saw references to Inkscape, which like GIMP is an open source product ("Inkscape: Draw freely.").
I downloaded and spent a bit of time with it and man! It's pretty nice. As with many free and publicly available software products, there are a couple of areas where you might wish for some additional polish or specialized support. But the quality of the editing and the basic toolset, not to say the drawing features that put it way above very basic drawing tools like OpenOffice, such as filters, extensions, paths, and layers, make it a fantastic drawing tool, and one I'd recommend heartily to authors.
Even if you're doing diagrams, simple boxes and arrows, the editing flow can be quick and nice, and it supports a bunch of output formats from its native SVG
editing, such as PDFs, ZIPs, PNGs (though some version of the PNG output lose transparencies and some layer details).
Image below, lame but the work of just a couple of minutes, shows an Inkscape drawing with some shapes, gradiants, connectors, and transparencies.