I wouldn't recommend anything inside imagej. I would however recommend Neuronstudio. It is a free open source program that has a simple user interface, and allows for manual, semi-manual and automatic tracing of dendrites. It will also quantify spine density and morphology. It is also able to perform a simple sholl analysis and branch order analysis on your tracing. I highly recommend it.
For more information about it and other programs you can use with Neuronstudio, I recommend you check out a recent blog I wrote about them all. It contains all the links to the programs as well.
No plug-in. The following paper could be of interest to you:
R. Su, C. Sun, C. Zhang, and T. D. Pham, A New Method for Dendritic Spines Detection Based on Directional Morphological Filter and Shortest Path, Computerized Medical Imaging and Graphics, 38(8):793-802, December 2014. http://dx.doi.org/10.1016/j.compmedimag.2014.07.006