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.