Probably a site most have heard of before, but well worth the look if you haven't. Basically, it's one man who sits at his computer with a slate and narrates videos explaining an idea/technique as demonstrated on a simple whiteboard. He's recorded a large compilation of these videos from a variety of fields in math and science. "The Khan Academy is a not-for-profit 501(c)(3) with the mission of providing a world-class education to anyone, anywhere. Despite being the work of one man, Salman Khan, this 2100+ video library is the most-used educational video resource as measured by YouTube video views per day and unique users per month."

This is a wonderful, cutting edge tool on the web for using the power of Mathematica to computer along with the endless databases of facts and figures that can be found on the internet at large. It can be a great resource for students, but it could also be something they use to cheat in certain situations so be aware. "Wolfram|Alpha is more than a search engine. It gives you access to the world's facts and data and calculates answers across a range of topics, including science, nutrition, history, geography, engineering, mathematics, linguistics, sports, finance, music..."

If you use Google Talk (Google's instant messaging client) or just chat from inside your GMail account, you can set up a HTML "badge" that you can copy on to any webpage so that visitors can just click a link and start chatting with you. I've used a badge copied on to my Blackboard site to allow students to instant message me. The advantages are that the student doesn't need to sign up for a Google account to use this, and they don't even have to know your gmail account address to chat with you since you can make the badge say whatever you like.