Gimp

The gimp provides different means for sharpening. However, High Pass Filter Sharpening gives a lot of control over the process and also quite impressive results

Vignetting is often an undesired effect from some lenses. But often pictures benefit from adding this effect.

There is also a plugin available for this effect: Vignette

Simple and effective way of enhancing contrast in images.

Gimp Plugins by me.