Open Middle (Challenging problems with multiple ways to approach and solve. They start and end the same way with opportunities for higher level thinking in the middle. )
NCTM - Annie Fetter (Let kids figure out the math involved. Here is a link to a 5 minute video for teachers about the prompts: What do you notice? What do you wonder? ) video
www.101qs.com (Questioning: Visit 101 Questions website to help students practice generating questions (appropriate for multiple domains). You can use the search option to help you select stimuli appropriate for your grade level...)
http://www.101qs.com/ddmeyer (Here is a link to Dan Myer's math prompts from this site...LOVE THESE! )
http://wodb.ca/shapes.html (Which one doesn't belong...see how many different ideas the kids can generate! )
Visual Patterns Link (Visual Patterns: Read THIS BLOG POST about an appropriate first lesson using these visual patterns first. Then use the link below for students using different patterns. )
Noetic challenge math (this is a sight with free samples of problems, you can also purchase more. They even have a math contest you can register with a group of students)
You Cubed (this sight is full of resources for full lessons or activities that can be used as warm-ups or centers)