Latest GIMP version 2.8 finally supports single window editing. I have yet to see it crash on Windows, but it works better on Linux (more features). It is NOT a Photoshop clone however, and has its own philosophy - I find its export capabilities better for the Web than Photohop's, with direct previewing and better settings in both PNG and JPEG formats.
Scribus is a very powerful editing tool that supports text flows, colour profiles for all elements and very advanced PostScript export capabilities among others. It is indeed professional level.
LibreOffice Draw is an integral part of the LibreOffice suite; as such, it CANNOT be downloaded separately. A large toolbox, powerful style habilities and nifty features make it very good for creating any kind of diagram which can then be ported to a LibreOffice Writer document. It was true in OpenOffice 1.1 ten years ago, it's even more true now.
Inkscape has powerful vectorization capabilities: import a raster image and vectorize it automatically, and you can soon stretch it in all directions. Automatic shape simplification will be a usefuil help then, lightening the result a lot (it removes extraneous nodes and manipulates the leftovers to keep the general shape) with barely any effort.
And then, the Blender community decided, a few years back, to ensure the tool is at a professional level by producing short- and medium-duration movies - and they rival some that are sown in cinemas these days. Their recipe is simple: if they want to render something and the tool cannot, they improve the tool. As the Sintel project shows:
http/www.youtube.com/watch?v=eRsGyueVLvQ&hd=1