Seems like I've read of past guest stars passing away several times over the last few months, so I thought I'd see if it was my imagination. Off the top of my head, these are the ones I can think of. I'm sure I've missed a few. Interestingly, many of the more "senior" characters are still with us: the "old bag" lady, Peterman's mother, and everyone from "The Old Man".
The Seinfeld Memorium list:
Actor Character Episode Birth/Death
Jesse White Ralph The Cadillac 1917-1997 Joseph Maher Gavin The Dog 1933-1998 Lloyd Bridges Izzy Mandlebaum Various 1913-1998 Billye Ree Wallace Nana Various 1925-1999 Ina Parker Mah-Jongg Lady #3 The Garage 1931-2000 Eric Christmas Haarwood The Gum 1916-2000 Sandy Baron Jack Klompus Various 1937-2001 Edward Winter Mr. Stevenson The Beard 1937-2001 Lewis Arquette Leapin Larry The Secret Code 1935-2001 Lawrence Tierney Alton Benes The Jacket 1919-2002 Benjamin Lum Mailman The Cigar Store 1953-2002 Sam Whipple Phone Guy #1 The Maid 1961-2002