Agent Research and Evaluation - Provides complete information on literary agents and their track record of sales for authors, often including dollar figures in the US, Canada and the UK.
Agents' E-mail Addresses - This is a list of literary agents' e-mail address and also agency websites if they have one. (ed: LONG list)
Everyone Who's Anyone in Adult Trade Publishing - A free online directory of almost 2,000 of the top literary agents, editors and publishers of adult trade books in the US, UK and Canada, including e-mail addresses and websites.
Preditor and Editors - A huge listing of agents and scuttlebutt on the black hats. If an agent has a bad reputation, you'll probably find the dirt here.
Publishers Marketplace - Provides membership information for publishing professionals and provides web pages for writers and agents to promote themselves.
Vetted, Non-Fee-Charging Agents - A list of vetted agents from author Natalie R. Collins who also wrote Buy This Book and I'll Wash Your Car: How to—and not to—Get a Literary Agent. Ebook for sale on site.
Writer's Market - A fee-based service which offers significant amounts of information on hundreds of literary agents in all genres. Also has market listing for magazines and publishers. The grandaddy of agent resources.
Writers Net - Not agents themselves but "a matchmaking resource for writers, editors, publishers, and literary agents on the Internet". Free service run by the husband of a writer.