A Replicator 2 build plate
My plate fractured as I was trying to desperately remove a part, so I had this one cut using a waterjet. I included the SolidWorks files in case you want to modify.
UPDATE: got a build plate waterjet from glass and it was one of two best upgrades to the Rep 2 and has had a substantial positive impact on my build experience. The other best is definitely this: http://www.thingiverse.com/thing:42250