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.