A few things you might find interesting (most of them things I like myself!).
More from journeytoberlin.com
More Berlin history
Architecture in Berlin - Jim Hudson roams Berlin photographing interesting buildings and writing about Berlin's architecture. Needless to say, I like this site a lot!
Aerial photos of East Berlin - Alain Guillou was taken up in a US military helicopter in October 1987 to get a unique view of the wall and East Berlin. He also sells canvas prints of his historic pictures.
Deutsches Historisches Museum - The German Historical Museum. Go for the information, stay for the webcams!
British Pathé - watch videos from the British Pathé archive online. Loads of Berlin-related material there, amongst other things!
Staying in Berlin
Mowitania Holiday Flat rentals - a nice way of staying in Berlin, usually quite inexpensive, especially for groups
Propeller Island - you could call it a hotel...but really it's more a work of art you can stay in.
Berliner Verkersbetriebe (Berlin Transport Company) - handy information for getting around the city by public transport.
Deutsche Bahn - their journey planner is very useful for planning train journeys anywhere in Europe. In Germany, it also gives you information on other forms of public transport (use the advanced search and put in the address you want to travel from/to).
Berlin Sketches - Eva Flug's impressions of the city. Some really interesting finds!
Slow Travel Berlin - helping visitors (and residents) explore Berlin the slow way.
And going off-piste a bit...
Old Newport Pembrokeshire - just to prove that some things run in families...