This site has searchable video cases of real elementary math lessons teaching standards based mathematics lessons. You can also see a complete lesson plan with handouts.
The NCTM website has excellent ideas for lesson plans, interactive on-line math activities, challenge problems, and more. NCTM offers many professional development resources such as journals, on-line conferences, information about regional conferences, an
This website lists a bunch of different interactive activities and is sorted by content standards, so if you are looking for a geometry activity you can click activity and a whole bunch will come up.
Great resouce for children's literature applicable to math lessons. You can search by grade or topic. The site also has links to some lesson plans and online activities which go with some of the books.
This link has many different videos and activities that work through many math topics. You can show only the parts you want, but it combines many ideas. The videos are not too long, maybe 10-15 minutes, to introduce topics.