There is always ORY-SXF and then SXF-MST (Maastricht, in southeast Netherlands). I believe Transavia, formerly BasiqAir, in addition to AirBerlin also flies AMS-BER. Note that EasyJet flies out of Berlin Schoenfeld and AirBerlin (for the AMS flights) and Transavia (at least they used to) fly out of Berlin-Tegel. Both airports are pretty easy to get to from anywhere in Berlin, Berlin has one of the best transportation systems in Europe. For alternate options, there is always Whichbudget which is a comprehensive directory of European LCCs and where they fly.

I really like EasyJet, especially since they opened their base at SXF. There is a good chance you will be on their new A319s which are brand new, but their older 73Gs aren't bad either.