Pink Monkey
Like Spark Notes, but MUCH better; affiliated with Barron's Booknotes; provides plot summaries, explanations of key elements. There are even message boards where people discuss various topics on their novels. Good stuff. REMEMBER: THIS IS FOR USE AS A STUDY AID ONLY! DO NO PLAGIARIZE, IN ANY WAY, FROM THIS SITE!!

Shmoop

STUDY AID ONLY! DO NO PLAGIARIZE, IN ANY WAY, FROM THIS SITE!!

The American Novel: Timeline
One of the handiest tools I've ever seen for placing American literature in context. Play around with it!!

Luminarium
Covers everything you'll ever need for British Literature.

Interactive British History Timeline
This goes crazy in-depth! Flash plug-in required.

Poets.Org from the Academy of American Poets
Build your own poetry notebook! There are also oodles of critical essays and related material on poets.