Of all the coffeeshops I sampled in Amsterdam this trip, Double Reggae was my favorite. It's located on a pedestrian street just off the Damrak. If you come out of Centraal Station along the Damrak, look on the right for Nieuwendyk and follow it along. You will see it in on the left in an area filled with coffeeshops, souvenir shops and pubs. There are also numerous gyro/french fry stands on the street, should you get hungry :-)
Double Reggae's theme, naturally, is reggae -- although the painting of Jim Morrison on the window threw me off a bit. I wasn't aware the Doors were considered reggae? The music is a selection of classic reggae (Marley, Tosh, etc.) along with some more obscure selections that will interest music fans. The shop itself is very dimly lit and narrow. The 'bar' is to the left and seating on stools and in booths is along the right and in the back. There are also bathrooms located in the back. The seating is not particularly comfortable -- no cushions or couches -- but is rather basic.
The reason I enjoyed Double Reggae is that it was not a coffeeshop full of hype or geared to gouge gawking tourists. The staff was friendly and helpful. The drinks (limited here to non-alcoholic selections such as coffee and tea) were fairly priced and there was no pressure to buy from their menu, which we did not. Rather, Double Reggae was a place where it was easy to spend a few hours chatting with friends, sipping tea and enjoying the day or evening. It was never crowded, that we saw, and was filled with a nice mix of locals and relaxed tourists. I will note that because Double Reggae opens early, it also closes before some of the other local coffeeshops.