While researching using manipulatives in high school math, I discovered a great article from Scholatic on using legos in the math classroom. The author states that when you first put out the legos, students will want to play, let them. Its always worth letting students check out the manipulatives or apps.
Many of the lessons, the author designed deal with part of a whole, multiplication and division, equivalent fractions, and my favorite - mean, median, mode, and range.
Each activity is well described, has lots of pictures and comes with already made worksheets that can be downloaded for immediate use. I grew up with legos being a toy, not as an educational activity. Who knew. So after reading this article, I did a search for more idea using legos in math and found the following.
1. love2learn2day has a lovely lesson using legos to help teach symmetry. It wouldn't take much to extend this to teach transformations.
2. Math with Legos by Lee De Cola has some lovely representations for sin, cos, radii, pi, bar graphs etc.
Legos are actually fairly easy to get second hand at garage sales. You don't need complete sets. You just need enough of the various sizes and colors to accomplish the activity. Unfortunately, there is more information out on Legos Robotics (which are great), not on their direct use in math as manipulatives.