You can find lots of information on the Semacode website - http://semacode.org/ - where you can not only download your copy of the reader software, but create your own semacodes using the Java applet on the site.
Below is a semacode for the AAS Mobile website:

A semacode is a small symbol that encodes a standard, web-oriented URL. The URL is embedded into a two-dimensional barcode along with error correction information. When the semacode reader software snaps the barcode, it launches the embedded URL on whatever web browser is available.