The tool I use for this in Inkscape is Filter Effects. Start by looking through the Filters menu, then try looking at how the stock filters were built using the Filter Editor and try altering them.

It's not exactly intuitive, and it can be VERY slow, but it is very powerful. Keep playing with it and you'll be able to pull of some very cool effects. I recommend you go into the preferences and enable multiple thread filter rendering if you have a multicore processor. Hopefully performance should improve in the next release or two, though it's still going to be a computationally intensive feature.