Sunday at 7 is a pretty low-priority timeslot, and much like all of Saturday, if you can find a mainstay, you keep it there. All of the big 4 have generally aired the same stuff in this timeslot across the three years that I'm using.
ABC: The net has gone for quite some time with mainstay America's Funniest Home Videos, which last year averaged about a 2.2 in originals. It will not be leaving anytime soon. The slot is sometimes used for NASCAR overruns or for the first half of two-hour Extreme Makeover: Home Edition events.
CBS: This network also has a mainstay in this timeslot, newsmagazine 60 Minutes, which usually gets major bumps in the fall from football overruns and was helped even more (as were all newsmagazines) by the election last year. The show averaged about a 2.7 during the '08-'09 regular season but because of the above reasons varied more wildly than any show in primetime, peaking at a 6.3 and bottoming out at a 1.5.
FOX: This timeslot is generally the home of football overruns (taken to 8 PM by postgame show The OT) and animation repeats. FOX occasionally burns off originals in this timeslot, most recently megabomb Sit Down, Shut Up which posted a paltry 0.7 on May 10, and game show Hole in the Wall which aired 8 originals in this hour in February and March of '09 and averaged a 1.1.
NBC: This is home to NBC's pregame show Football Night in America which has always been fairly low-rated. Some of the explanation for this is that there's always a game still going on against it and that ESPN recently switched from a 7 PM Baseball Tonight to an NFL edition of Sportscenter. It returns this year. Dateline NBC has historically done most of the lifting when football season is over and should continue to this year.
CW: To put it kindly, the CW has had a lot of variety in this timeslot. The network launched with a two-hour comedy block, starting with originals Everybody Hates Chris and All of Us in this hour, but moved the entire block to Monday and brought over dramas 7th Heaven and Runaway. The America's Next Top Model encore that had previously followed the comedy block filled in at 7 until the CW burned off the final season of Reba in the slot and aired Reba repeats for the rest of '06-'07. In fall of '07 the CW tried to get into the newsmagazine business with CW Now and Online Nation. Both shows struggled to find even a million viewers and Online Nation lasted only three weeks before getting yanked for a comedy repeat. In 2008 the CW leased the real estate to Media Rights Capital and aired Discovery Channel-esque In Harm's Way to almost equally disastrous results. The CW took back the time and filled it with repeats of failed CBS series Jericho for most of the rest of '08-'09 and eventually gave up on the whole night, returning it to the affiliates for local programming for fall of 2009.