The latest great science web resource I found is BioEd Online and its companion site, K8 Science. The sites have news, presentations, slides, lesson plans, and teacher resources for biology. While the sites cover relatively few topics in biology, they still have a decent repository of good quality resource available for free.
The sites have a lot of information about laboratory techniques, including a sheep dissection video, measuring and counting with a light microscope, micropipetting, dilutions, and more. K8 includes some physical science topics as well. Despite their limited subject coverage, they have managed to put out quite a bit regarding embryonic stem cells; unfortunately they have no information regarding other sources of stem cells. They have a presentation on cloning, too.
Overall the sites are a great place to look for ideas or to supplement something you may be doing already.