The only place in Amsterdam you're going to find great music, creative live acts and lots of great costumes on Halloween is at Boom Chicago. They are an American comedy club on the Leidseplein. I've been every year for the last 3 years - and plan to go again this year! Although I am a HUGE Halloween enthusiast, my more timid friends enjoy the parties too. They have different themes every year and have a great crowd of expats, tourists and locals. More info is on the website at www.boomchicago.nl
This year the party is on Saturday the 29th of October.
I was there for the Halloween week-end and the town was definatly buzzing with lots of people dressed up, really good fun, we did different bars and clubs. I would definatly recommend Halloween week-end for your trip