Gridlock Clamp

Specialized clamp to join two 1.5" pipes at a 90° angle. Particularly useful (and less expensive) than cheeseboroughs or when the pipes cannot be separated to allow a part of the clamp in between.

http://www.jrclancy.com/catalog_crossgridconnector.html

Another manufacturer:

http://www.thelightsource.com/products/11/view