A classic in the large format is graphic designer Kit Hinrich's 365 Typography Calendar. He used to make it for Pentagram when he was there, but now that he's struck out on his own they're under his own name. Each month is dedicated to a new typeface, but the real thrill is the sheer size of the thing. I'm partial to the large one (it measures in at 23" x 33") though if you've got less wall space then the smaller one (12" x 18") will also do the trick.
A newcomer that I also like quite a bit comes from the New York graphic design firm Studio Newwork. The Newwork Calendar is more like a poster, and the point of departure is the musical staff that dominates paper made for musical notation.
The calendar is really big at 26" x 40" and they only printed 100 of them. I have number three at the office so you might well stand a chance at getting one of your own. They cost $48.