Current Sox who have World Series rings earned while with teams other than the Sox:
Mike Timlin (1992-3, Toronto)
John Olderdude Olerud (1992-3, Toronto)
David Wells (1992, Toronto; 1998, Yankees)
Edgar Renteria (1997, Florida)
Matt Mantei (2001, Arizona)? (Not in the postseason if I recall right)
Curt Schilling (2001, Arizona)
Current Yankees who have World Series rings earned while with teams other than the Yankees:
Gary Sheffield (1997, Florida)
Kevin Brown (1997, Florida)
Randy Johnson (2001, Arizona)
Tony Womack (2001, Arizona)
Carl Pavano (2003, Florida)
Alan Embree (2004, Sox)
Current Yankees who were in teams that won postseason series against the Yanks:
Tino Martinez (1995, Seattle)
Alex Rodriguez (1995, Seattle)
Jaret Wright (1997, Indians)
Alan Embree (2004, Sox)
Luis Sojo (1995, Seattle), if you count coaches