I discovered the AIMS Educational Foundation last year when a coworker donated a large stack of their books to me.
For all of the assault on the convention floor of buzzwords, such as "standards based", "research based", "promoting critical thinking", very few products actually deliver on those claims. AIMS products consistantly do.
Unlike ordinary worksheets, their projects have a extensive lesson plans which cover not only all the basics of the lesson, but which concepts it brings together, what understanding the students are supposed to develop, and ways of extending the lesson once the basic concept has been established.
They provide solid educational resources all the way through algebra and geometry. Their problem solving projects (part of which is embedded above) have become a staple for me on the days before vacation, when the kids are starting to tune out, these projects get them involved and interested in mathematical thinking. The projects differentiate nicely, and a lot of them are appropriate for a wide range of grade levels.
The have a magazine which provides continuous new material (4 times per school year, based on a traditional schedule) in 4 major age spans.
They also have a new service, E-Activities which allow you to buy individual activities rather than a whole books worth. Nominal cost is $2 per activity.