Always heartwarming to see, if the match is close enough to make it meaningful. Latest (Munich, 2009-5-5, as reported in a post on AndyMurray.com):
However this account is disputed: some say umpire overruled & Hewitt argued with that. (I didn't see it.) More below.
Hewitt was serving for the match against Petzschner and had MP, Petz goes long but Hewitt contests it and gives him the point. Petz goes on to break back and take the 2nd set
A classic from Roddick in 2005 which actually did cost him the match, as reported in Sports Illustrated
Last Wednesday, in the round of sixteen at the Rome Masters, Fernando Verdasco of Spain was serving to America's top player, Andy Roddick. Verdasco had lost the first set and was down 5-3, love-40 -- triple match point. After he hit deep on the second serve, the line judge called the ball out and Roddick had the match.
Only, Roddick refused to accept the point. Verdasco's serve had nicked the line, he said. Stunned, the umpire let Roddick overrule him. Verdasco then fought back, held serve, won the set and then the match.
Probably you heard nothing about this display of generosity; it barely rated a mention in the American press. Yet Roddick risked -- and lost -- tens of thousands of dollars in a tournament where he was seeded first simply because he felt obliged to be honest.