There was a post earlier asking about the names of George's parents, so I did a little digging into "The Entertainment Weekly Seinfeld Companion", the FAQ you always wanted and often asked for, and found the following:
George's parents are Estelle and Frank Constanza, currently played by Estelle Harris and Jerry Stiller. Prior to this past season, Frank was played by John Randolph.
Jerry's parents are Helen and Morty Seinfeld, currently played by Liz Sheridan and Barney Martin. Phil Bruns played Jerry's dad in the second episode only.
Kramer's mother, who has been mentioned but never seen, is named Babs Kramer. You know what that means, don't you? That's right, Kramer is Kramer's *last* name. Live with it. This is the first of two clues, BTW. The second is when Kramer says he would like to name his child Isosceles Kramer (if he ever has a child).
I can't find any reference to Elaine's parents. I'm pretty sure she has some. :-) Anybody have any additional information?