For prototyping,
Esto Connectors work really well, are reasonably priced (they also sell the square tubing on the website) and I've always gotten quick turnaround from them. We've used it for some framing on competition robots but generally it turns out to be too flexible for anything under load (especially dynamic loading). But still, the joints can be built up with the plastic connectors and then replaced or reinforced with shear plates once the design is firmed up. The tubing is very lightweight.