My new favourite truck in town is without a doubt Van Leeuwen. Although it's famous for it's ice cream (which is quite amazing) I have been going there for only one reason...the coffee! They are serving up some of the best mobile cappucino's around. They are brewing the very famous LA-based Intelligentsia coffee and espresso daily. I follow the truck on twitter and can plan my route to work based on the position of the truck at the time. Before 10am, it is located on 6th Ave and 20th, then after 10am it rolls around the corner to 23rd street, closer to 5th Ave... right in front of my favourite lunch spot, Mangia! So I usually hit it up once in the am then again after I grab lunch! This is a GREAT alternative to Grumpy's, which is a good 10-15 walk away.
Van Leeuwen Artisan Ice Cream
(at a few select locations around the city)