In my office it is a Greenpeace calendar that I got from a sister as a birthday gift. Very beautiful pictures (January's picture is three Emperor penguins standing in front of a massive glacier/cliff in the Antarctic).
At home I have a calendar of airplane history that the Smithsonian puts out every year. My partner gave that to me as a birthday gift.
I got a wall calendar from the power company, but I probably won't use it. Every year, I buy a pocket sized 'week at-a-glance' and keep it on my desk, opened to the current week. For electronic reminders, I use the alarm feature of the Gorgy Clock widget.
I bought the Mormons Exposed calendar...with the hot Mormon guys. I heard about it from a RJ Forum post, lol. Tho, I think I might have to get the Naked Frat Guy one, too. I looooove naked frat guys, lmao.