Does a good Latin-Latin dictionary exist, for student use?
I know Comenius wrote one a few hundred years ago aimed at students, but I can't locate an online scan of it. Once a student passes a certain stage, they should not be using a bilingual dictionary. An all-Latin dictionary becomes another text to read.
Maybe someone out there should write and publish one, as I don't think there is a modern one available?
There are a few options:
There is the far from complete Adumbratio Lexici of David Morgan, at http://alpha.furman.edu/~dmorgan/lexicon/adumbratio.htm .
There is the Latin Wiktionary. I cannot say either how comprehensive or how accurate this resource is.
There is DuCange's wonderful (and rather large!) Glossarium, available at: http://standish.stanford.edu/bin/search/advanced/process?sort=title&browse=1&clauseMapped%28creatorBrowse%29=Du+Cange%2C+Charles+Du+Fresne%2C+sieur%2C+1610-1688.
DuCange's text could, in principle, be reprinted fairly easily---although it is a massive text and would be expensive.
Another idea, taken from Justin Bailey, is to just create your own Latin only lexicon, as you learn words. This has advantages and disadvantages, of course.