Jared Richardson, author of Ship It!, has a standard e-mail with links which he sends to people who ask him "What is agile?". Here are are some of the links:

The start of Agile
The Agile Manifesto

An overview of the many Agile flavors
The New Methodology by Martin Fowler

XP and Scrum are the most popular flavors of Agile, so here are a few links specific to them.

A comparison of XP to Agile by Bob Martin.

An intro to XP

Scrum info on Wikipedia.

Found at Jareds blog Agile Artisans.