At hackerbot last weekend, they set up a Cloud Chamber, which I attempted to get a good photograph of. This is about the best I got, though I feel like there's a lot more that could have been obtained... But I don't know much about it all, so maybe not? Anyway, here it is.
The rock is Torbernite (thanks, Æther, for filling us in!)
Random note: including private uploads, this is my 888th flickr upload. Kind of a fun number. :-)