Postponed

Due to the shooting in Connecticut some shows were cancelled or postponed.

Haven: Reunion 
Originally took place during a high school reunion and had some violence. The SyFy channel left these statements: "In light of today’s tragedy in Newtown, Connecticut, Syfy will not be airing the new episode of #Haven tonight at 10/9C.", "At this time, no decision has been made as to when the episode will air."

General Hospital: 12.14.2012
The episode was postponed both on ABC and Soap Network due to the coverage of ABC news. They plan to air the non-controversial episode on Monday.

Most of ABC and the rest of networks pre-empted their daytime shows but since they are horrible it didn't really matter.

Over at the laist they have a great article of other shows and events that were pulled, I helped out a bit with one tip.