Finding ways to clamp onto pipe provides lots of options: kee klamps, speed rail, lighting c-clamps or mega clamps, or even leg sockets for platforms.
Berge Equipment offers several styles of clamps for square tube (1.5"), including a flange. Most of the clamps connect a pipe or tube to a square tube for railings.
The light Source has clamps for 2" box that are similar to cheese boroughs.
Palay Display carries a clamp that allows you to insert a 5/8" threaded rod on the top for construction specialty racks.
Wagner offers square tube flanges and flange covers.
For casters, Service Caster has square sockets for stem casters.
And for capping the tubes look to alliance plastics.
NRG Researchoffers a clamp that is reminiscent of a beam clamp. Once again you are limited to a 5/8 stud or receptacle for hanging from the clamp.
Also check out J W Winco for more options and their blog. The blog, by the way is actually very informative and fun - definitely worth a read.
Shape Products has a variety of square tube components (curved pieces, corners, flanges)
And for splicing two tubes together take a look at wagner.
For legs, the Leg-a-Matic II can be used with either 2" square or 1 1/2" sched. 40 pipe.
I suspect that the options are greater for round tube / pipe because it is more difficult to weld since you have to shape the round tube to match correctly (though this this is easier with the correct tools, or you can purchase shaped connectors to weld on).