We added a particularly interesting aluminum liftarm today. This beam has one flat end and is threaded on this end. This lets you attach it to any standard Technic-compatible part at a perpendicular direction to the existing beam. Using this new part, we can now build L and T beams of varying lengths.
The other really neat thing you can do is fasten the end beam at any angle of rotation with respect to the original beam. Because the end is threaded and the main beam is not, the included low profile cap screw captures the main beam between the screw head and the threaded hole in the end beam leaving you free to leave the end beam at any point of rotation you desire.
Finally, if you attach it at through the hole in the apex of an existing L-beam, you can build a three dimensional corner with liftarms extending on the X, Y and Z axes.
Here is an example of a gear box build from two of the end beams and a few standard LEGO™ parts.