The museum is named after one of its star exhibits, the Pergamon Altar's sculpted frieze which originally ran around the temple to (it is thought) Zeus at Pergamon.
The extant sections of the 113m-long frieze are displayed surrounding the museum's giant entrance hall on three sides, with the full-size altar steps (leading up to an area displaying another frieze) and collonade on the fourth.
Crappy Youtube-res Wobblevision™ video to give you an idea of the scale of the room:
Also cool is that you're allowed to photograph & video as much as you like in there (flash off, of course) – wish all museums were like that!
But even more impressive than the Pergamon Altar is the massive Ishtar Gate and accompanying Processional Way. These massive artifacts were created in Babylon about 575C and later partly destroyed and partly covered.
They were excavated and partly reconstructed in the 1930s – the Museum shows the smaller of the two gates (the bigger one would be absolutely enormous – look at the size relative to the people in the photo of the gate, then look at the model in the fourth photo) and a number of panels from the Processional Way.
And there's lots, lots more. Beautiful sculptures of several traditions, fine mosaics with exceptionally subtle use of multiple color shades, ancient red-figure Greek vases, Cuneiform tablets, coins, interment vessels, columns…
And this is just in the main section! I didn't even make it into the other sections, such as the Islamic Art section. Next time.
Entry to the Pergamon Museum is free on late night Thursdays and at other times costs 8 EUR for a day pass (which includes all the other state museums). Highly recommended.
Definitely invest the few Euro for the audio guide – it is very informative and adds a huge amount to one's appreciation of the stuff. And it has an option to lead you on a short highlights tour if you're pressed for time.