GeoGebra: Free, open-source, multi-platform mathematics software I often use to graph functions. I find GeoGebra useful to double-check my own sketched graphs or to get information about a problem that would otherwise be difficult or tiresome to calculate algebraically. I primarily use the Mac desktop software, but there are tablet versions and an online one too.
Grapher: A graphing tool bundled with Mac OS X (10.4 or later). Search for it using Spotlight or check the folder Applications->Utilities.
Quick Graph: A third-party graphing calculator app for the iPhone and iPad. Recommended for those who cannot afford or do not wish to pay the $100+ for a real-world graphics calculator. It’s free for the basic version and $2.79 USD (c. 2016) to enable its advanced features.
Paul’s Online Math Notes: Created by Professor Paul Dawkins of Lamar University Beaumont, Texas, Paul’s Online Math Notes has everything serious high school students need. His notes, examples, and cheat sheets are beautifully laid out, comprehensive and clear (I especially love the Trig cheat sheet). Topics range from algebra basics to Calculus III.
Khan Academy: A comprehensive and well-organized video-tutorial website for math and other subjects.
Ask Dr. Math: Using HTML technology no more sophisticated than when it was created in 1994, this website contains questions emailed from students around the world that are answered by math experts and presented in a friendly, clean, plain-text Q&A/email-conversation format.
Wikipedia: For better or worse, Wikipedia is typically the site I automatically go to when I’m curious about something. Sometimes the articles are helpful, but the level of detail and jargon often leaves me feeling lost. A good resource to dive deeper, but not necessarily the best place to start when learning a new topic.
Wolfram Alpha: I use Wolfram when I need a formal definition or explanation. But, as with Wikipedia, I sometimes get blurry-eyed from all the jargon and abstract explanations. The look-and-feel of the site is pretty cool, however, and apparently you can use it to solve math problems too.