ActiveBass.com: Interactive lessons with tablature and music playback, tab search, tracks, reviews, forums, and learning aids.
Bass Survival 101: A collection of lessons on bass-playing, created by Dale Titus.
BassLobster: Online lessons, MIDIs, software, videos, and links.
StudyBass.com: Bass lessons covering theory and technique. Includes audio examples, photographs, video, and printable fretboard diagrams.