This website offers exactly what it sounds like - everything Emerson wrote, online. It is run by the Ralph Waldo Emerson Institute. His essays, poems, lectures and other papers are neatly archived for your scholarly use. Think of it as a lighter, more portable anthology of Emerson's complete works.
This Emerson-centric site has online versions of his works. By far the coolest feature is a tool that lets you search Emerson's texts for a specific word or phrase. Make sure Emerson actually said that pithy quote you found on the Internet before using it in your paper!
This great site from Virginia Commonwealth University offers an overview of the Transcendentalist movement in America. It is a really helpful resource if you're trying to understand the broader context of how Transcendentalism came to be, and what it gave to America. A hyperlinked breakout page focuses on Emerson.
This is the official homepage of the Ralph Waldo Emerson Society, which celebrates its 20th anniversary in December 2009. As you can imagine, it contains a wealth of information on Emerson's life, plus writings by and about him. The list of Emerson souvenirs dating from his time is entertaining.
Reuben, an English professor and Emerson scholar at California State University at Stanislaus, has compiled this interest page of Emerson-related links and miscellany. Some of the biographical information is student-written and should not be cited, but it's useful as a reference for Emerson works.
Emerson was nothing if not a philosopher. His ideas shaped American culture and the way we see ourselves. Emerson's entry in this online encyclopedia outlines his philosophies. It's an incredibly helpful summary of the intellectual themes of a man who could be expansive to a fault in his speeches and writings.