Extreme Programming

• Belgian XP Website
• Dutch XP Website
• Agile Systems
• Benelux XPDay
• Agile Manifesto
• Agile Alliance
• Martin Fowler's Refactoring pages
• Refactoring Thumbnails
• Test Driven Development

More Agile sites:

• Mary Poppendieck's Lean Software Development
• Ken's Schwaber 's way to control chaos: Scrum

...and related techniques, methodologies, lines of thought:

• Systems Thinking
• Retrospectives
• Fixed Price Agile Projects: check out Pascal Van Cauwenberghe's articles The price is right and Do you want agility with that?