If you have a photo that could stand for a little artistic flair, Psykopaint is a feature-rich, web-based virtual painting tool.
Source image by freebird.
Psykopaint is definitely not a traditional editing tool, so if you're looking to make more standard image adjustments, check out previously reviewed SUMO Paint. Psykopaint is focused exclusively on recreating paint effects and paint textures.
You can fine-tune the brushes you use by setting the minimum and maximum width of the brush, so each brush stroke will be unique and vary in width based on your parameters. You can also adjust the highlights and shadows for the brush strokes, the blending mode, and how strongly it will mix the colours it pulls from your source photo. Psykopaint currently limits images to 1000px by 1000px, and will scale any image you upload to fit within those constraints.