This article is from the NYPD Blue FAQ, by Dave Chapman, Alan Sepinwall and Jeff Knapp with numerous contributions by others.
A better question might be, which haven't? Here's the rundown
of regulars and semiregulars who have appeared in the buff at
least once: David Caruso, Sherry Stringfield, Amy Brenneman,
Dennis Franz, Jimmy Smits, Sharon Lawrence, Gail O'Grady, Kim
Delaney, and Justine Miceli. The only cast members to have
never dropped trou are James McDaniel, Nicholas Turturro, and
Gordon Clapp. Guest stars who have appeared naked include
Debrah Farentino, Melina Kanakeredes and John Wesley Shipp.