Here are some of the places I go for technical information:

Google Scholar
Search engine for scholarly articles. Pros: does a good job at filtering out blog posts and reviews and "help!" stuff. Cons (for many): most of the articles it finds are for pay.
NIST Chemistry WebBook
A free authoritative collection of chemical, reaction, thermodynamic, and other properties, courtesy of the National Institute of Standards and Technology.
The javascripty thing that makes math display actually work on the web. To go along with it, the LaTeX math commands it supports and LaTeX math reference (pdf) so I know how to make it show what I want it to show.