Orange - Open source data visualization and analysis (ailab.si)

From what I hear this is a pretty powerful data mining package (developed at the faculty I attend). Personally I haven't gotten to play around too much with it, never had the time to figure it out properly.

Protip: use it as a python library, a lot of stuff doesn't make it into the GUI.

Speaking as a rubyist, now that's a compelling reason to use Python more.

I would be happy to use this to read some MongoDB data as well (I guess it's probably possible).