i'm very choosy when it comes to (a) whether or not to buy a special-purpose ruler or template; (b) which one to buy.
I know there are many good brands on the market, but i'm particularly fond of the Wright's EZ ruler/template line. I found their Tri-Rec tools on sale today at Joann's. the package contains BOTH tools needed to take the guesswork out of those pesky tall triangles within squares (like the ones in the 54/40 or Fight block (http://www.quiltingboard.com/posts/list/23653.page).
To my surprise, i discovered that Joann's regular price for the set is lower than the other sites i searched. Right now, they're on sale for only $6.99 a set. even after tax and shipping i saved money. (now, really ... how often does that happen?)