I just found this online article How to choose your Yiddish dictionary by Jordan Kutzik. It is oriented towards English speakers but nevertheless gives a decent overview of some of the available Yiddish dictionaries.

Those who read French, however, should consult the superb Dictionnaire Yiddish-Français by Yitskhok Niborski and Bernard Vaisbrot which is considered by many to be the best Yiddish dictionary available today. It can be ordered through the Medem Library.

For those who do not know French the good news is that an English version is currently in preparation, supported by the American Friends of the Medem Library. More information can be found here. An interesting article from the Yiddish Book Center's magazine Pakntreger discusses this project and provides a short history of Yiddish lexicography as well. Links to available online dictionaries will soon be added to Yiddish Sources.