FunBrain - math games, including math baseball and mathcar racing
Awesome Library - has lesson plans, games, etc. Grouped by grade level. Not just math, and not just elementary.
AAA Math - Search by topic or grade level - quizzes that keep track of the number right & wrong, and of the time spent on the task.
PBS Kids Cyberchase - Lots of puzzles, games. Very colorful, good looking site.
Maths Is Fun - Games and quizzes.
ABCteach - makes worksheets, puzzles. Not just limited to math.
Mathrealm - for grades 3 and up. A lot of the stuff here is for sale, but many activities are free, and have very cool graphics. Includes tangrams.
Superkids - creates worksheets instantly
Visual Fractions - A tutorial that models fractions with number lines or circles.
CoolMath4Kids - "An amusement park of math and more." Includes lemonade stand.
BrainPop - Movies and activities. Not just limited to math. You can only use it twice a day for free.
Curious Math - lots of tricks, number games, brain teasers
Learning.com - registration required, but it's free. For K-8th grades. Not limited to math. Includes the movie on grouping and labeling.
KidsDomain - math games with sounds, moving graphics.
Count Us In - games for basic number concepts
Carolyn White's Tangram Files: