Kartograph is quite nice. I need to play around with it more.
However, it's aimed at a very different audience (web-based interactive rendering of vector graphics, as opposed to desktop-based interactive rendering of scientific data). For example, as far as I know (I could be very wrong about this), there's no way to display image/raster data in kartograph.
That having been said, kartograph is a very slick library, and its built-in clustering methods for point data are particularly nice.
