When you are working with pipe it is a real chore to thread the end to accept standard threaded couplers and flanges. Also, you have a limited array of fittings available. Enter Kee Klamp and Hollaender. Both have a wide array of fittings that slip onto the pipe and tighten down with a set screw. They are great, totally reusable, and worth the investment (it is however, and investment as they can easily run 25 each, meaning that to build a first project can run you a couple hundred to start out). There are engineering specs available for both so they can be used for load bearing structures.
Also, looking at what they use them for is a good way to get ideas.