Inkscape is an indispensable tool for me. I use it for quick drawing and drafting, for presentation slides, illustrations, small-scale print works, and even just pictures for fun. It allows to combine freehand drawing and moving things around with very precise, CAD-like handling of shapes, sizes, coordinates, etc.

There are few tools that are very ingrained in my daily operations, stay for years, and would be hard to replace, like Emacs or Firefox; Inkscape is among them.