There is an enormous amount of information regarding Differentiation available. Although you could spend hours and hours searching, here are some of the sites I think are pretty good:

Hoagies Gifted 

The MOST comprehensive site I have ever encountered!  There are literally hundreds of pages - tons of information and resources.  Fantastic pages devoted to lesson plan ideas and student activities!

The Math Forum

Drexel University's Teacher 2 Teacher resource for teaching math at all levels -

Math Playground

Great site for elementary/middle school math games, puzzles, programming, and logic!

Kathie Nunley's Layered Curriculum

Ms. Nunley's site includes information about her "Layered Curriculum" model (Tiered Instruction) as well as examples of units/lessons.

 

Bertie Kingore's Site

Ms. Kingore's selection of web resources regarding Instruction, Assessment, Grouping, Thinking, Literacy/Reading, and Gifted.

 

Not really DI, but great sites about Thinking:

Components of a Well Developed Thinking Skills Program

Dr. Arthur Costa's article is insightful and thought provoking. ~If you are interested in the "Habits of Mind", more information can be found on Costa's & Kallick's website.

The Thinking Classroom

Another of my favorite sites dedicated to enhancing Thinking in the classroom.

 

**Just Found:

Virtual Field Trips - Links to tons of Virtual Field Trips to take with students!