This is a customizable L-bracket for connecting 20x20 (or similar) aluminum extrusion. You set its dimensions based on your hardware parameters (width of your extrusion, length of your inserts, and your screw size) plus the number of holes in each section. To use, you will probably want to print two per corner, with the second copy mirrored across the Y axis. One goes on top of the joint, and the other on the bottom.
If you need a "T"-shaped bracket you can find that here: http://www.thingiverse.com/thing:1876275