Until recently I was using a 1940s diner coffee cup that I found at an estate sale for a nickle.
It finally broke all over my cubicle one day 
My college friend Laura sent me a wonderful Miracle Over Baltimore mug: it’s a cityscape of Baltimore, but when you put hot liquid into it, an image of Pope John Paul I appears hovering over the sky, spreading his blessings on all the happy Catholics!
It’s bliss, and also fun to put ice cubes in and make the Pope disappear.
I grab whatever is handy. Usually it’s a mug with the Community Coffee logo. (I’d post a pic, but they don’t seem to have it on their site.) This morning, since it was in front, I’m using a cup from Café du Monde. (I found an image.) It says, ‘Peace, Love & Beignets’. It was in a gift basket roomie got me for last Christmas. Behind the two Café du Monde cups and the Community Coffee mug are a couple of Los Angeles Independent Film Festival mugs, and of course a Straight Dope mug.
I don’t drink coffee, so my favorite mug doubles as a nail, tack, brad and such receptacle in the cabinet behind my desk at work. In any case. it’s an over-sized mug with the Air America logo on it.
Mine says “Institute for Applied Ethics”. I got it at an intercollegiate debate meet I was judging a year ago. I love judging that event and it reminds me that the next one is right around the corner!
Staff
The Straight Dope
Fighting Ignorance since 1973
(It’s taking longer than we thought)
And a Slug illustration, of course.
If there was a mug that said WARNING: CONTENTS MAY BE POISONED, I’d buy one.
Mine has fat happy Christmas snowmen all over it. It was a gift, but the reason it’s my favorite is because it holds about three cups of coffee.
I have two favorites, amongst all my many coffee cups(which I all like!)
One has the Babylon 5 logo on it, and a picture of the station.
The other has a picture of a coffepot on a pedestal, with people bowing down to it in worship.
If you pick it up with your right hand, it reads: You are dumb.
If you pick it up with your left hand, it reads: The Onion.
I never really think about what is on my cups until someone comes over, gets served a steaming mug of fuck (or whatever I’m serving at the time) and says, “Are you trying to tell me something?” Sometimes their feelings are genuinely hurt. I laugh at those people.
Full of the milk of human kindness, I am.
I have a mug that says “Do not even THINK of putting coffee in here!”
It’s from a tea shop and my S.O. knows that I mean what it says or he will buy me new mugs.
It must make for interesting weather down in Maryland.
“We’ll have a clear Pope through the mid-afternoon but temperatures will drop in the early evening so expect intermittent holiness through the night.”
My favorite mug was an old Pirates of the Caribbean mug I got a Disney World. But I broke it and I can’t get a replacement - all the souvenirs they sell now are based on the movies.
It’s BadBob holding a gun, and it says “What part of ‘no decaf’ don’t you understand?”
Actually a beer mug I use for coffee because it holds a lot more than a coffee mug…
From the Trails Inn Bar in Miles City, MT.
“Where the Beer Is Colder Than Your Ex-wife’s Heart”
National Synchrotron Light Source
Brookhaven National Laboratory
Associated Universities, Inc.
With the BNL/AUI logo and the NSLS logo. It was a freebie from a conference when I was working at NSLS. An irreplaceable reminder of my brief career as a scientist. It does not get much use since I don’t want to risk damaging it.
I also love the mug Mama Zappa gave me that shows a nightscape which, when your pour hot liquid into the mug, turns into a sunrise. 
Liquid crystals are awesome! I have a similar mug that shows a map of the world. Pour hot liquid into that wnd it shows the post-global warming shorelines. The last two are not dishwasher safe, so they don’t get much use either.
Mine doesn’t say anything but is a perfect replica of a TARDIS. People who aren’t familiar with Doctor Who are always shocked to see a square mug :p.
Alaska Highway - 65 Years 1942-2007
French Market - New Orleans
It would be cool if you had a cup with a heat-sensitive TARDIS on it. Then when you put coffee in, it would disappear. (Dad had a Stealth mug that did that.)
North Korean flag and text “Welcome to Pyongyang”