The website radicalmath.org has a lot of great math social justice lesson plans. They sometimes need a little tweaking for the grade level. It seems to me that math has more lessons that necessarily build on each other so it's harder to have a collection of math lessons that you could drop into any class.
I've also been able to find some stuff at curriki.org but there is more junk to wade through there.
Though they're not quite lesson plans, quizville.com and funbrain.com have good math activities/games that you could base a lesson around using either a computer lab or a computer connected to a projector.
The math sister site to ReadWriteThink is Illuminations, which is based at NCTM. Thinkfinity is the umbrella that sponsors similar sites across the content areas. The Math Forum is another popular math resource site for educators.
Another great site for math--as far as being interactive for the kids would be the National Library of Virtual Manipulatives. Its one of my favorites to use with my projector.
Though it's not comparable to ReadWriteThink yet, MathWire has a lot of potential. The focus feature changes each month, but there is an archive that let's you access everything that has been presented to date.
I was going to suggest Illuminations, but I see that has already been done.
Thanks for highlighting resources from ReadWriteThink. We are very proud of what we have to offer on the site! If you are interested, we pay educators in the field to publish lesson plans and share teaching ideas. Let me know if you would like more information.