Hindustani
For the twinned-histories of Hindi-Urdu, see Shamsur Faruqi's A Long History of Urdu Literary Cultures in Literary Cultures in History.
Online Dictionaries
Originally published in 1929 by the Hindi Nagari Pracarini Sabha, this dictionary-translated as the "Ocean of Hindi Words"-spans 11 volumes and has been digitized for online searching by the Unviersity of Chicago. It remains the most authoritative Hindi dictionary.
A Dictionary of Urdu, Classical Hindi, and English
One of the most used and important dictionaries (includes entries in both Devanagari and Nasta'liq scripts). Online at DSAL.
A New Hindustani-English Dictionary
Online by DSAL, originally published in 1879
Full list of online dictionaries at DSAL