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Introduction
Geogebra is a multilingual freeware for teaching and learning with dynamic geometry, algebra and calculus. Geometric objects and algebraic expressions are manipulated in an integrated environment. This is a Java based software and can be accessed through common browsers. Its author is Markus Hohenwarter (from the University of Salzburg, Austria). The official website is http://www.geogebra.org/cms/.

Major support materials include quick start and help, user forum and wiki, all available in the help section of the official web site.
link to quick start
link to help (online or pdf)

Other resources
an introduction given by Julie-Ann Edwards and Keith Jones, accessed via
Investigating Geometry with Geogebra at Henrico County Public Schools
Geogebra applets provided by Saint Louis University (SLU)
Manuel Sada's mathematics web site (in Spanish)

Here are some screencasts (videos) explaining some techniques in Geogebra, produced by Linda Fahlberg-Stojanovska.
 
Here are 4 blank dynamic worksheet for quick trial of graphing and construction online.
geometric construction on a blank sheet
geometric construction on a square grid
graphing with grid and axes
2 graphing windows on the same page (you may use it, for example, to compare 2 graphs of the same functions with different scales)