Is there a way to download additional brushes and filters for Inkscape similar to how you can on GIMP?

Additional brushes?
Like these?

http://en.wikipedia.org/wiki/Vector_graphics

Inkscape, as a vector editor, doesn't really have brushes. You can create effects that look like brushes, but it's kind of the long way around, compared to using a raster editor and drawing with a brush.

OpenClipart is a good place to search for filters for Inkscape, although there won't be many (i don't think). But definitely look up ivan_louette. Another member here, v1nce, also has made a lot of filters, but I don't know if he has them on openclipart, or if he has, what username to search for. There are a few of his scattered through the forum, so you could search the fo rum. I think I heard that the upcoming new stable 0.91 has a LOT of new filters, and you can get it now as experimental. (inkscape.org) (The release is getting pretty close, it sounds like.)