Inkscape

This screenshot by Aaron Spike demonstrates the multiscan tracing feature new in 0.41. You can select between brightness, monochrome (both b/w), or color, then set the number of levels, and optionally smooth the bitmap before tracing. You can also choose between stacked (paths overlay one another, leaving no gaps) and non-stacked (paths run edge-to-edge) options.

Apart from the symmetry group, you can vary the shift, scale, rotation, and opacity of the clones in the pattern, per row and/or per column. Any of these parameters can also be randomized to any degree. This produces "object fields" that can be arbitrarily distorted, skewed, slanted, "faded out," or "magnetized" like iron particles in a magnetic field. The creative potential of this new feature is enormous.

"What immortal hand or eye could frame thy fearful symmetry?" The new powerful "Tile clones" dialog implements all 17 plane symmetry groups, allowing you to easily create all sorts of symmetric patterns, Escher-like tesselations, ornaments, etc. Since the pattern is made out of clones, you can freely edit or add objects to the original tile, and the entire pattern is updated live. (Source files: art-nouveau-P3.svg, eastern-motive-P4G.svg, tesselation-P3.svg.)

The Da Vinci Secret is a beautiful piece of Inkscape artwork created by Radoslaw Turek. The starry sky was done with the help of the new "Tile clones" dialog with randomized placement, scale, rotation, and opacity. Visit the author's page for the SVG and PNG versions of the image, as well as an interesing walk-through showing the process of creating the artwork step-by-step.

Jakub "jimmac" Steiner uses Inkscape for creating Gnome icons. Snapping everything to the pixel grid (using the new px unit) minimizes the undesired antialiasing blur at the target size, while going to 1:1 zoom (press 1, then ` to return to previous zoom) is a quick way to preview how exactly the icon will look when exported or rendered.