From 25C3 Public Wiki
 Venue Location
The Congress takes place at the Berlin Congress Centre (BCC) in the East centre of Berlin - right next to Alexander's Square Alexanderplatz (commonly known as Alex) - a busy place with many shops and an intersection of many buses, s-bahns, trams, and underground lines (u-bahns).
Berlin's tallest landmark, the Fernsehturm (TV tower) is close to the square, making the Alex an easy place to find from most of Berlin.
If you are new to Berlin, you should get a city map and a map of the local train system. You can get maps of s+u-bahns, underground and metro buses at many BVG shops for free, and city maps are available for a few euros.
bcc Berliner Congress Center
Alexanderstr. 11, 10178 Berlin
 Alexanderplatz Station
The Alexanderplatz train station can be reached by buses, trams, S-Bahn, U-Bahn and regional trains. Basically, ask anyone "how to get to the Alex".
 Arrival by local transport
The bcc Berliner Congress Center is located southeast of Alexanderplatz. Leave the train station at exit Alexanderplatz and turn right, pass along the World Time Clock and proceed to the pedestrian crossing at Grunerstraße and cross Alexanderstraße afterwards. Then you should be standing in front of the bcc.
 Arrival by train
Probably the best way to travel to Berlin by train is from Hanover, Nuremberg or Hamburg via the ICE (InterCityExpress) routes. The trains stop at Hauptbahnhof station. Look up your connections with the Online Booking System of the Deutsche Bahn.
 Arrival by plane
 Tegel Airport (TXL)
By bus only:
from: Tegel Airport via: bus TXL unto: Alexanderplatz time: 37mins
in words: Take the bus TXL unto S+U Alexanderplatz Bhf/Memhardstr.. The ride should take 37mins.
 Airport Schönefeld (SXF)
Since Schönefeld is in Zone C you will need a ABC-zone Ticket ("Berlin ABC") which needs to be validated before travel! This airport has a direct connection to Alexanderplatz via S-Bahn S9 (directon of Spandau) and also by Regional trains RE7 or RE14.
from: Airport Schönefeld via: s-bahn S9 (x:13, x:33, x:53) unto: Alexanderplatz time: 41mins
from: Airport Schönefeld via: RE7 (x:47) or RE14 (x:17) unto: Alexanderplatz time: 28mins
in words: The s-bahns leave every 20mins (at 13,33,53) and take 41mins unto Alexanderplatz.
The regional trains come every 30mins - the RE14 at x:17 and the RE7 at x:47. Each train takes 28mins until Alexanderplatz.
You don't want to take bus to Rudow or the new bus service to Südkreuz. Südkreuz is far of the bcc and Rudow even farer.
 Tempelhof Airport (THF)
You're a real hacker landing on that airport! ;-)
 Arrival by car
You must buy an emission-rating decal "Umweltplakette" for your car to be allowed driving within the Berlin center (within the s-bahn ring).
Also, parking space here costs quite some money, too. Usually two euros per hour, up to three euros per hour. There is no free of charge parking space anywhere around the bcc for at least half a kilometer!
The garage at the Alexa shopping mall has many places and is open from 07:00 to 24:00 - fee 1 Euro per hour or 10 Euro per day.
However, not for all areas of Berlin are parking fees charged, so its probably the cheapest way to park the car either northern of the "Jannowitzbrücke" (U8) or eastern of the Strausbergerplatz (U5). Both of these two locations are one Subway-Stop away from the bcc, so its only a "Kurzstrecke" (1,30€ per direction).
This wiki has a special page for announcing and requesting lifts to and from the congress: Traveling groups
But you can also check out the following sites:
MitFahrZentralen (MVZ) - Overview: http://bellnet.de/suchen/reisen/mitfahrzentralen.html
http://drive2day.de aka http://hitchhikers.de free of cost :) http://mitfahrzentrale.de http://mitfahrgelegenheit.de http://mifaz.de