Plenty of space, a great atmosphere and free wifi make this an awesome place to get some work done in the afternoon or just hang out with friends later in the evening.
Think Phil or Möbel, but with superb and cheap food (limited to 2-3 new and unique choices each day). They also host interesting cultural events from time to time.
I've only recently been here after hearing about it for a while, and I'm very impressed. The atmosphere is simply awesome: the architecture together with the wall projections really turn it into a surreal place. This has the potential to become one of my favorite spots in Vienna.
Vienna's oldest Crêperie (that's why they got to call it just that): They serve pretty awesome crêpes until just before midnight in a somewhat plain, but very friendly atmosphere. It's not too cheap though: a crêpe costs around 4-6 euros, and you'll want to have two.
Minimalistically furnished bar/club near the Flakturm with plain walls, electronic music, and lots and lots of art students. It sometimes doubles as a gallery. A little hard to find, but worth the effort.