ImageJ has great image stack tools (slicing, manipulation, analysis). Has the "tubeness" measure from the diadem challenge.
ImageJ Cookbook
The "cookbook" plugin for microscopy seems to be very useful in particular.
MBF Plugin Collection
The original software associated with the "ImageJ for Microscopy" reference (see below). It seems defunct now, and is supplanted by the ImageJ Cookbook plugin suite.
Sezgin & Sankur. 2004. Survey over image thresholding techniques and quantitative performance evaluation. Journal of Electronic Imaging, 2004
For reconstruction and analysis of dendritic spines: Rodriguez A., Ehlenberger D.B., Dickstein D.L., Hof P.R., Wearne S.L. Automated Three-Dimensional Detection and Shape Classification of Dendritic Spines from Fluorescence Microscopy Images. PLoS ONE 3(4): e1997
For reconstruction and analysis of dendritic arbors: Wearne, S.L., Rodriguez, A., Ehlenberger, D.B., Rocher, A.B., Hendersion, S.C., and Hof, P.R. New Techniques for imaging, digitization and analysis of three-dimensional neural morphology on multiple scales.
Neuron Datasets
NeuroMorpho
Seems to be an open neuron reconstruction dataset. They don't appear to have the original images, unfortunatley. I could try to contact the labs