This is a good hostel with friendly staff.
We originally checked into a hostel right outside the Zoo Station because it was so convenient, but it was quite crowded and a little bit faceless. A friend recommended the Heart of Gold, and we moved there the next night.
It's further away from the main station than some of the other hostels but it's worth making the effort to get there. It's in a great location for lots of the sights (e.g., Reichstag, Alexandersplatz, Brandenburg Gate). It's quite a quirky place with a real atmosphere. Music's pumping out the whole time in reception, and there's some crazy decor (of particular note were the Barbie doll in a cage and the Elmo amidst bottles of spirits).
It was generally a nice hostel: big and spacious and immaculately clean. They have an Internet connection (probably more expensive than EasyInternet but obviously more convenient), they give a good breakfast (served till 12 noon, so there's no hurry), and they serve drinks at reception.
All in all, this is a worthy hostel.
Website:
http://www.heartofgold-hostel.de
PS When we told the manager about our around-the-world project, she even gave us some free T-shirts!