Reference: dictionaries, encyclopedias and more!  Tags: dictionaries encyclopedias atlases  

Merriam-Webster: handy for a quick word search.

Visual Merriam-Webster: A dictionary with pictures in addition to definitions.

Oxford English Dictionary: the ultimate in word definition!

Foreign language dictionaries, e.g. English-Spanish: go to the Oxford Reference Online site, they have a wide range of bilingual dictionaries.