It seems like I am always looking for a corner extrusion of some sort, so I thought I would start collecting some of that information.
Orange Aluminum has a variety of “Z” clips (or panel clips). I found them because they have corner channel mold pieces that they market for retaining panels on road cases. Its for ½” or 3/8” material though. They also have a “valance” profile that makes panels lock together like a tongue and groove panel would. The have divider bars – something that I could being used instead of quarks for where panels meet up together. They also have some edge trim that could be used for counters and desks. It looks like it might be better than t-molding, but would not be flexible.
Outwater, of course, has a variety of plastic and aluminum corner channels.
Wgsonline sells a variety of glass products and has corner extrusions for ¼” glass. Though glass is not critical, they could be used for other panels. However, you can buy vacuum cups / section lifters if you want to move a heavy glass panel around.
US Plastic Corp has a clear two way corner channel joiner. They also have just about everything from bags, bottles, buckets, fittings, pipe, tube and sheets… if its plastic, they might have it.
Extrude a trim (which calls the corner molding F-channel) has a variety of sizes and a couple finish options. They also have other trims as well. Also stock F channel, (and H, h, T and Z) is Brunner Enterprises. Brunner also carries square tubing connectors for ¾” and 1” tube.
Also check out Paramount Extrusions.