Math specific stuff
My hands-down favorite collection of math resources is http://illuminations.nctm.org/. I know many of y'all might already be familiar, but I have to pitch it just in case. The site has lessons organized by topic and grade (click "Lessons" on the left for a search screen.)
Also highly recommended is Sam J. Shah's virtual filing cabinet (part of his Continuous Everywhere.. blog). The virtual filing cabinet includes links to good teaching resources for algebra II and above. Link here: http://samjshah.com/worksheets-projects/.
The next overall website is http://nzmaths.co.nz/. This is a website created by the New Zealand Ministry of Education, so the website is organized according to New Zealand's education system. Our 8th grade is roughly their level 4 or 5.
Different blogs are good for different grade levels, but my favorite is "I hope this old train breaks down", link here: http://untilnextstop.blogspot.com/. The author posts quite frequently and links to all sorts of great stuff. I read it for inspiration as well as specific lessons - I like her teaching philosophy.
Also good are dy/dan, Continuous Everywhere but Differentiable Nowhere, f(t), and many others. I highly encourage you to look at the blog links on the sides of the above blogs and bookmark/rss the ones you like depending on what classes you teach. Links: http://blog.mrmeyer.com/, http://samjshah.com/, http://function-of-time.blogspot.com/. For an example of a valuable blogroll, see http://myweb20journey.blogspot.com/p/expanded-blogroll.html.
There are tons of sources of good problems for your math classroom, here are two: Mathcounts (middle school) and Exeter math (high school). Also, check out Alcumus and For the Win! from Art of Problem Solving as well as agmath.com. Kuta math software has free worksheets on a big list of math subjects,which are so-so. Site: http://www.kutasoftware.com/free.html.
For those of you that have projectors or access to computers but not Geometer's Sketchpad, check out Geogebra. It's basically the free, open-source knockoff of GS, and it's very, very similar, though it lacks the polish of Sketchpad. Some links: http://www.geogebra.org/cms/ for download, and http://www.geogebra.org/cms/en/help has a bunch of how-to files.
Lastly, our professors are really smart people and they do a lot of great research into teaching and learning. Here is a good research-based primer for math teachers: https://www.ncetm.org.uk/public/files/224/improving_learning_in_mathematicsi.pdf.
Get dropbox or use google docs. Dropbox is cool because you can share your lesson plans and files with all your GSE friends, and that can become a hugely valuable resource. This year's math kids didn't do it, but the English kids did.
If you use google docs you might as well set up a google reader for all your math blogs - which you read religiously, every day, right after flossing! I set up categories in my google reader for "functions" "graphing" "statistics" "awesome" etc and whenever I read an inspiring article I tag it for future use. Set up some sort of system for organizing cool things you find.
Check out the Khan Academy: http://www.khanacademy.org/. You might as well get familiar with this site, which has around 2,100 videos that explain math, physics, chemistry, history, finance and some other stuff, as well as a growing library of computer-checked problems to go along with each topic. I used this a couple of times to email videos to my students who missed class, and I think its a great resource!
For videos, see also: http://www.brightstorm.com/math. Additionally, you get tons of hits for videos on a given math topic by typing something like "mixture problems" into youtube.
Odds and ends
Math for Primates is a great podcast by two professors from Portland: http://www.mathforprimates.com/. They went on hiatus since one of them is writing a book called Punk Mathematics (!) here: http://www.kickstarter.com/projects/1541803748/punk-mathematics
There are some really great science simulations and applets created by the University of Colorado here: http://phet.colorado.edu/. Also science-y, check out http://www.withouthotair.com/, which is a math/physics based look at sustainable energy. Lastly, check out http://www.youtube.com/user/engineerguyvideo, who explains how things work and the engineering behind them. Awesome!
Hopefully this post has been helpful to you. I'd really like to hear what resources you have found most useful in your teaching practice. If you have a great resource, post it to the comments, or put it on your blog and send me a link. Cheers!