Reply to comment
I've been a fan of GeoGebra since I saw a demonstration of it by Markus Hohenwarter as MSRI last year. It's a free (open-source), cross-platform (Java-based), dynamical geometry and algebra application.
The nice part about being java-based is you can export your GeoGebra worksheets to html with embedded java applets. And then with a bit of parameter munging you can upload them to your blog. The embedded applet can be as function as the application itself (a nice advantage to being free; it can't be stolen so there's no barrier to making it completely available).
The nice part about being open-source is that people can contribute to it as much as they want. Now in the works is a tool to export a GeoGebra worksheet to PGF/TikZ, so you can put them in your LaTeX documents.
Here is an example GeoGebra worksheet, in which a regular pentagon is constructed using only compass-and-straightedge techniques:
The whole thing is based on the OP line segment. Drag either of these points, the circle changes, and the construction with it.
To get the TikZ code, select File > Export > Drawing Pad to PGF/TikZ... You can export as a stand-alone file, generate the code to copy and paste elsewhere, or generate the code straight to the clipboard.
Apparently this is pre-release feature, but I'm looking forward to using it.