I have a couple of clamping squares – they are useful for making sure that corners are perfectly square when I do glue-ups.  I have one from Woodpeckers Precision Tools.  It is a beautiful piece of machined aluminum:

I love the the tools from Woodpeckers.  All the ones I have used are high quality and precise.  Not the cheap tools one can get from the local stores.

I have two larger plastic clamping squares from Rockler.  They are plastic, and of course don’t have the fine feel of the machined aluminum.  However, they are a little larger (and easier to clamp), and work just as well for the projects I have needed them for.  They are also a little cheaper:

I only have 3 total clamping squares and have been able to get by with that many.  Another 2 would be useful at times.  I might recomment starting with a pair of the aluminum ones, on sale now at Woodpeckers.  The Woodpeckers tools are made in the USA as well.