Spencer Magnusson's free Light Painter lets you interactively paint the surfaces you want lit and then places the light for you.
The new 1.2 release adds the following features:
- Live preview for all tools (ie see the final position and shape of lights as you paint)
new keyboard shortcuts while using tools, allowing parameters to be adjusted live. - Includes shortcuts for lamp offset, size and power
- Sky Paint tool: a sky texture's sun size and intensity can now be adjusted with its size and power parameters
Download it for free from Github.