Helpful Software
Download Adobe Reader (to open PDF documents)
Download Geogebra (for graphing)
Download Mathtype (for creating formulas, free trial then turns into light version)
Online Equation Maker (for creating formulas, no install required).
Screenr (for recording short screencasts)
Ubermix (Linux OS that AHS uses on netbooks)
Grab (built-in to OS X for taking screenshots)
Snipping Tool (built-in to Windows 7 for taking screenshots)
Screenshot (built-in to ubermix Linux for taking screenshots)

Calculators (let me know if you have additions or recommendations)

 Physical Calculators
TI-36 Pro (Scientific, ~$18)
TI-84 (Graphing, ~$95)

Downloadable Computer Software Calculators
Mac OS X and Windows 7 both have built-in free calculator apps. (Go to View and choose Scientific to get more options.)
Microsoft Mathematics 4.0 (Windows)
Solution (Mac OS X)
CalcMadeEasy - Free (Mac OS X)
Extcalc (Linux)

Online Calculators
Desmos Graphing Calculator (also pretty decent scientific, click "show more" on the keypad to get additional functions)
Wolfram Alpha
(Also see Chrome Web Apps below)

Calculator Phone Apps
Graphing Calculator (iOS, $1.99)
PCalc Lite (iOS)
RealCalc (Android)
Scientific Calculator (Android)
Algeo (Android)

Chrome Web Store (Apps - many free - to install into your Chrome web browser. All apps below are free.)
Numerics Calculator and Converter (after installing and launching, click the little keyboard on the upper right to get the calculator keypad)
Graphing Calculator
Daum Equation Editor (great for creating math equations to screen capture for blog posts, Moodle, etc.)
Awesome Screenshot (great for getting screenshots)
Aviary Image Editor
Aviary Audio Editor
Advanced Periodic Table