Grade Level 
Grades 14 enrichment. This site also has problems for middle and high school students that may be appropriate for your most advanced students. 
Problem Solving 
There are a wide variety of problems at this site. They are creative and authentic, attractive to children's interest, and foster higher order thinking. 
Description 
This site allows you to pick an age group, content area (based on NCTM standards) and type of story problem and then offers many problems under each one. The site also offers links for teachers such as a rubric for coding the problem difficulty, a chance to read what other teachers say about how they use the problems in their class, and a link to look at student solutions. The students have an opportunity to send solutions to the site and mentors from high school math classes across the country will respond to the solutions and communicate with the students working on the problems. If the problem is solved correctly, the students will be recognized on the site.
The problems of the week are categorized into the areas of elementary, middle, geometry, algebra, discrete math, and trigonometry & calculus. Under these headings they are further broken down into different areas. For elementary there are topics such as algebraic reasoning, patterns, logic, probability and statistics. Under story problems, topics include age, animals, food, travel/geography, holiday, sports, and money. Each problem is also rated on a difficulty level of 15 according to the NCTM standards. 
Appeal and User Friendliness 
This site does not have the "bells and whistles" such as animation and graphics that other sites have but is very well organized and easy to use. The links described above are rarely found in other sites. These are useful for differentiating instruction within the regular classroom and for providing additional challenge to bright students. 
Sample Problem 
This problem was found under elementary (35) algebraic reasoning with a difficulty level of 3.
A man purchased what he thought were 6 barrels of soda, only to find out that one of the barrels contained apple juice. (He loved apple juice!) The barrels held 15 liters, 19 liters, 20 liters, 16 liters, 31 liters, and 18 liters. He sold some of the barrels to Randy and some to Rachel. Randy purchased twice as many liters as Rachel did. Being very fond of apple juice the man kept this barrel for himself.
The problem is to figure out which barrel (in liters) contained the apple juice. Of course the man sold the barrels as he bought them, without taking a drink from any of them, so tasting didn't help him figure this one out. 
Website 
mathforum.org/library/problems 
