Digital handbook of mathematical functions with formulas, graphs, and mathematical tables. Managed by the National Institute of Standards and Technology.
CDE has been around and evolving since 1978. This online version contains over 25,000 technical terms and phrases, 3,000 images, and 40,000 cross references, It covers all aspects of computing, from chips to supercomputers and from command examples to mini-tutorials. It's free and there's no advertising.
Webopedia is an online dictionary of computer- and Internet-related terms.