THDL Sanskrit Dictionaries Overview
There are numerous online Sanskrit lexical resources, most of which are digitized forms of older dictionaries. The most important is the classic Monier-Williams Sanskrit-English dictionary under the rubric of the Cologne Digital Sanskrit Lexicon. A useful interface for running this dictionary offline can be found at: http://members.ams.chello.nl/l.bontes/sans_n.htm. It allows for rapid searching and viewing of the devanagari script. Install the beta version, fonts, and dictionary; also, be sure to put the beta (mwsdd.exe) program and the dictionary (.txt) files in the same folder and don't forget to install the fonts.
The Practical Sanskrit-English Dictionary of Vaman Shivaram Apte is also available.
Capeller's Sanskrit-English Lexicon
Yashwant Malaiya's search interface for Skt dictionaries
Amarakosha: a Sanskrit thesaurus written by Amarasinha in verse format. He was a distinguished scholar, one among the nine "gems" (navaratna) during Vikramaditya's court (in about 380 A. D.)