Berlin is the capital of Germany. wikitravel.org provides nice informations about what you can do with your stay in Berlin.
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There are two airports in Berlin, named Tegel (TXL) or Schönefeld (SXF).
From Tegel Airport, take the bus TXL driving to "S+U Alexanderplatz" up to the station "U Turmstrasse", then walk through the small park to the other side of the bus station to take the M27 to "S+U Pankow" or the 123 to "Invalidenpark", depending on which comes first. You have to get off in Quitzowstrasse, and after getting of the bus, turn left to follow the bus' direction, but immediately turn right into the "Lehrter Strasse". Then you're almost there, just a few steps until on the right, there's the IN-Berlin. For this ride, you just have to pay one full fare, which is 2.40 Euro (AB ticket).
From Schoenefeld Airport, you have to pay full fare but an ABC ticket, which is 3.10 Euro. You can either take the RB14 (driving to "Nauen") or RE7 (driving to "Dessau, Hauptbahnhof" or "Bad Belzig, Hauptbahnhof") up to "Berlin Hauptbahnhof". Then follow the description of how to get there via train.
Coming by train, you'll most probably end up in Berlin Hauptbahnhof (Berlin main station). When you are there, get out of the North entrance of the Hauptbahnhof, walk over the street and get into the 123 driving to "Saatwinker Damm, Maeckeritzwiesen". You have to get off the bus in "Krumme Strasse" and follow the bus' direction, but when you have a playground on the right, switch to the other side of the street, walk a few meters further, until on the left you have the IN-Berlin.
There are several trains connecting Berlin directly with many other European and German cities. You can use the excellent online ticketing system of the Deutsche Bahn to book a ticket: bahn.de
When you take the bus to go to Berlin, you'll most probably end up in the Central Omnibus Station (Zentraler Omnibusbahnhof). When you are there, follow the signs inside the station to the entrance where you still have to walk a few meters to get to the S-Bahn station "S Messe Nord/ICC". Going down there, you have to take the S41 which is driving in a ring in clockwise direction (!), i.e. it doesn't have a final destination. Do not take the S42 driving against clockwise direction, you'll have to drive 3/4 of the ring instead of just 1/4. You get off the train in Beusselstrasse and there, get out of the train station and take the 123 driving to "Invalidenpark".
You have to get off in Quitzowstrasse, and after getting of the bus, turn left to follow the bus' direction, but immediately turn right into the "Lehrter Strasse". Then you're almost there, just a few steps until on the right, there's the IN-Berlin. For this ride, you just have to pay one full fare, which is 2.40 Euro (AB ticket).
Different to the rest of Germany, Berlin has many bus lines connecting it to the rest of Germany and Europe. You can look for cheap fares on berlinlinienbus.de.
When you come by car, you have to somehow get to the conference site: OSM.org.
As the Lehrter Strasse is usually crowded, you should look for a parking lot in the nearby Kruppstrasse.
There are three hotels/hostels nearby. Though their distances are different, they are all close to the conference site, maximum 15 minutes by foot or three bus stations (you don't have to take the full fare, but short fare for 1.40 Euro).
The Motel One is the most expensive hotel, prices starting at 69 Euro per night.
The closest location is the AO Hostel, also with prices ranging from very cheap (for six-bed-dorms 9 Euro per night) up to a bit higher for single rooms (starting at 29 per night). It is one bus stop closer than the other two.
The Christian Gästehaus also provides cheap rooms and is appr. as far away as the Motel One, starting from 22 Euro for a 6-bed dorm up to 34 Euro for a single room.
The conference site is the space of the IN-Berlin e.V., a local community-driven Internet Provider in Berlin.
It is barely visible from the street, you have to watch out for the shopping windows to get the right place. Though it is located in the Lehrter Str. 53, you should not go to the house entrance but look for the front entrance of the space.