In response to the "favorite online learning tools," I offer some great sites that I found useful in my teaching of science at a small, private high school.
To help keep my bookmarks organized, I love Symbaloo: http://www.symbaloo.com/
A very useful collection of math and science lessons are found at Kahn Academy: http://www.khanacademy.org/
Wikiversity often offers more than Wikipedia: http://www.wikiversity.org/
I don't know why it took me so long to find Stellarium, a free map of constellations, planets, stars and galaxies that is updated in real time for whatever location (e.g., Boston, MA or Kalispell, MT) you enter: http://www.stellarium.org/
Surprising fun can be mined from a website dedicated to warning people of the dangers of dihydrogen monoxide at http://www.dhmo.org/ Everything on the site is factual (except for the serious tone) and it leads to a great discussion on the importance of knowing a little about science and the meaning of "excess ingestion" of any chemical.
The Concord Consortium (http://www.concord.org/) has a wide variety of science simulations.
I use Photoshop as a hobby, but I find it useful for teaching when I need a specific graphic. There are many places to learn basic Photosop, including Photoshop Magazine and their website (www.photoshopuser.com). But a good, free site for a beginner is http://www.myjanee.com/.
Science Netlinks (http://www.sciencenetlinks.com/), Scientific American (http://www.sciamdigital.com/index.cfm?fa=Main.ViewMain), and Invention Dimension (http://web.mit.edu/invent/index.html) are broadly useful for science teaching.
Of course, the National Science Teacher's Association has great resources: http://www.nsta.org/
I teach some history of the sciences and enjoy http://www.chemheritage.org/ for background on chemistry.