Exploratorium

A cool site for science teachers, Exploratorium.com has lots of online activities to play around with and if you're close enough (San Fransisco), it is also a museum that looks to be a great field trip option.

Online Calculators

These definitely aren't tools that I will likely use in the classroom, but they might be useful to anyone teaching math or science.

Check out web2.0calc by clicking here.
Check out eCalc by clicking here.