Each month displays a different piece of artwork that just so happens to have a small calendar on it. I love that it's more art than calendar. Of course, you can always trust me to prefer form over function (in my personal life, that is...I try not to make my clients suffer in this way!).
But for a second choice, I could easily go with this one, also from Paper Source:
All the nostalgic images from Italy are really fun...especially since I have a bit of a soft spot for Italia. They also offer a New York calendar, with vintage travel posters, and a vintage Paris calendar. But if none of these are your style, check out this one:
It's full of prints of vegetable drawings from the 19th century. Kinda kitchen-y and cool. And for my more romantically-inclined friends, look at this one:
Aren't these botanical illustrations beautiful? Celebrating flowers without being fussy. I really like that.