The Agile Alliance is the best starting point for anyone interested in learning more about agile software development.
Agile Model Driven Development (AMDD) is described in detail here.
My January 2007 column, Agile Testing Strategies, describes how agilists go about testing their systems.
The article System Use Cases overviews how to write use cases in several different styles from point-form to very detailed.
Introduction to User Interface Flow Diagrams describes how to draw high-level diagrams which explore the UI architecture of your system.
Introduction to Acceptance Tests as Requirements Artifacts describes acceptance/customer tests and shows how they are an important type or requirement specification.
Introduction to Test Driven Development (TDD) overviews the process behind TDD.
Agile Models Distilled provides links to overviews of a wide variety of models.
The Principles of Agile Modeling v2 are described here.
The Practices of Agile Modeling v2 are described here.
Check out the Agile Modeling mailing list.