Lernu.net
A great, fun site with many courses to choose from.
Has a dictionary always available; you can click on a word within any course, and the dictionary will show the definition!
Edukado
Edukado.net
Unfortunately, this site is in Esperanto, which isn't practical for new students. But after a little practice, you can join the site and download several textbooks!
ELNA
Esperantousa Bookstore
This is the bookstore of the national Esperanto organization (USA). Textbooks, CD's, DVD's, etc. Members receive a 10% discount.
Esperanto HyperCourse
Hypercourse Index
A wonderful click-through course, though the sound is in '.au' format. (Sun audio)
