The Mauerpark ('wall park') is so called because it is located in the dead-zone of where the Berlin Wall ran through.
On one side of the park is a large flea market open every Sunday, and though the park itself is a bit run-down (what park in Berlin isn't?) there are a few really neat spaces. One of these is the large stone arena where each Sunday from May-October a guy comes and sets up speakers, his laptop, and opens up the stage for karaoke. Better known as 'Bearpit Karaoke', it borders on being too awesome for words. Well... once the drunken bums stumble down from the stage, that is.
At the market, you can find pretty much anything. A used bike. A brand new alpaca sweater. Handmade ceramics. Screen printed T-shirts. Housewares that look like they might have spent the last 60 years in a dusty attic. Tasty sausages.
People often complain that the Mauerpark market is "too touristic" [one of my favorite ESL-isms] but, given that well over 80% of tourists in Berlin are actually from other parts of Germany, I'm not sure what they mean by that. You know people. Once something becomes popular with the masses it's just not cutting edge enough for them. They probably yearn for the time when they had to barter for bananas through underground passageways, so don't listen to them. Instead, just join the touristic masses and enjoy the fact that you can spend a whole Sunday sitting around drinking beer and still feel like you've accomplished something.