William writes: “These ‘engineer’s squares’ from Gladstone are metal squares with a heavy base. They’re great for making sure your saw blades are square and the small ones fit well into tight places.” Gladstone also claims they’re accurate to +/- 0.0005″ per linear inch for squareness.
They’re priced quite reasonably, too: the four-piece set in a wooden presentation case (pictured) runs only $30, and a six-piece set’ll only set you back $65. These would make great gifts.
For those of you looking for different brands, Google Product Search turned up a number of other options in the same price range.