Table of Contents
Dictionaries and Thesauri
Dictionaries
English
Present-Day English
The Oxford English Dictionary (Online)
A large dictionary of Present-Day English on historical principles, based on citations from a large range of written published English. Includes the first known use of every word sense. Access: Free access from the campus wifi (or via VPN connection) Link: http://www.oed.com/
For historical/diachronic linguistics, the timelines and advanced search functions are especially helpful. It allows you to filter the entire OED for words first attested in a given span of time, borrowed from a specific language or family of languages and/or belonging to specific semantic fields.
Merriam Webster's
The oldest dictionary of American English, based on citations from a large range of published written English. Includes information about etymology, but is not built on historical principles. Access: Free Link: https://www.merriam-webster.com/
Earlier Stages of English
Bosworth-Toller
Online version “created by the labour and valour of many, maintained by Ondrej Tichý at Charles University in Prague”
Clark Hall and the Germanic Lexicon Project
Clark Hall is another dictionary of Old English, hosted by the Germanic Lexicon Project. CH Germanic Lexicon Project
The Germanic Lexicon Project by Sean Crist also hosts a range of other resources for the study of Germanic languages whose copyright has expired, e.g. Cleasby & Vigfusson's Dictionary of Old Norse Icelandic.
Pokorny's master etyma (if for some reason you need to know the PIE root)
German
Digitales Wörterbuch der deutschen Sprache
Access: Free Link: https://www.dwds.de/
Duden
Access: Free Link: https://www.duden.de/
Deutsches Wörterbuch von Jacob Grimm und Wilhelm Grimm
Access: Free Link: http://woerterbuchnetz.de/cgi-bin/WBNetz/wbgui_py?sigle=DWB
The Wörterbuchnetz (Trier Center for Digital Humanities, Universität Trier 2008) also contains, among other dictionaries, the digital Lexer (Mittelhochdeutsches Handwörterbuch), a dictionary of vulgar Latin and several German dialectal dictionaries. The full list is here: WBNetz
Thesauri
English
HTE, the Historical Thesaurus of English
Especially useful for (historical) synonym studies. Allows you to create wonderful timeline visualizations like this one:
With links to the OED.