Rich math tasks are a great way to address the CCSS Standards of Mathematical Practice while building content knowledge. They're also a wonderful formative assessment tool.
There are a number of resources available as sources of great, engaging mathematics tasks.
Michigan-developed MAISA CCSSI Units (Michigan Association of Intermediate School Administrators - Common Core State Standards Initiative)
The MAISA project has taken the CCSS-M and CCSS-ELA standards and placed them in units of study for all grades K-11. The mathematics units include a unit plan, a detailed model lesson from the unit, one or more formative assessment tasks, and a wealth of other resources. The units are made available through MAISA's Atlas curriculum management software's public site.
Mathematics Assessment Project - from MARS (Mathematics Assessment Resources Service) - Middle and high school mathematics lessons and tasks are available.
NRICH - Enriching Mathematics - from the University of Cambridge
Ohio Resource Center's Problem Corner
National Council of Supervisors of Mathematics - NCSM is publishing in 2013 a new version of its "Great Tasks for Mathematics" (original set of problems released in the 1990s).
3-Act Math Problems - An innovative way of approaching and enriching mathematics problems; many sites are noted in this LiveBinder.
The Illustrative Mathematics Project - Click on the "illustrations" option to view a number of tasks available.
Resources to help plan for and implement these great math tasks for teaching and learning:
Thinking Through a Lesson Protocol (TTLP)
TTLP Planning Guide
The Thinking Through a Lesson Protocol
Be sure to read the original TTLP article published by NCTM:
“Thinking Through a Lesson Protocol: Successfully Implementing High-Level Tasks.” Mathematics Teaching in the Middle School 14 (October 2008): 132-138.