Thanks! What other items often go together with these aluminum extrusions? Is there a place or a book to learn about this topic, or is it something that people only learn through experimentation and mimicking?
Primarily they are used for building structures quickly and easily - a saw and a wrench are the only tools you need. The standardized brackets for each beam type allow you to make 90 and 45 degree angles.
But they are often used for more than just framing. The 3D printing community has embraced extrusions because you can also use them as bearing surfaces, mount motors and servos, limit switches, etc. Basically anything that has a hole big enough for a machine screw can be mounted to a beam either directly or through an easily made mount (usually to get the angle that you want - all it takes is some sheet metal).
The quickest way to learn is to look at examples. The OpenBeam website has lots of examples. The system is so simple that you can understand exactly what is going on just by seeing a picture.