Geometer’s Sketchpad is a useful tool for visualizing euclidean constructs, as well as a multitude of other things. However, it is also expensive, with a new license running around $70, and renewals at $50.
Luckily, there is an open source version of the software called GeoGebra.
There are a number of links to help you get started with this free software:
http://www.geogebra.org/cms/en/download Here you can download the software for installation on your computer (this is how I use the program).
http://webspace.ship.edu/msrenault/tutorial/index.html Here, one may tour the GeoGebra software via a well put together tutorial.
That’ s all I have for the moment. A quick Google search for GeoGebra will pull up all kinds of reference materials