When was the last time we signed a former NBA Champion?
Actually an interesting question in the literal sense - meaning ignoring whether Rondo might be a good signing or not.
There are probably three ways to answer this question.
1) The technical answer
To the best of my ability to tell, the most recent player that the Kings signed as a free agent who at that time had an NBA championship ring was Melvin Ely, in 2009. However, Ely was not signed to a year-long contract - he was signed to a training camp contract and did not make the team. Furthermore, while he was traded to the '06-'07 champion Spurs at the deadline, he did not play a single minute in the playoffs.
2) The guy who played a bit
We signed Beno Udrih in 2007. He was a member of both the '04-'05 and the '06-'07 Spurs. In '04-'05 he played 11.5 minutes per game in the playoffs. So he was a useful bench piece on a champion.
(I am also counting here Chucky Brown, who averaged 15.5 playoff minutes for the '94-'95 Rockets but played only a total of 92 minutes for us in '01-'02 and Bill Wennington, who averaged <10 playoff minutes for both the '95-'96 and '97-'98 Bulls but only played a total of 57 minutes for the '99-'00 Kings.
3) The most recent guy who definitely counts
Vernon Maxwell was the starting SG for the '93-'94 Rockets, and when he signed with the Kings in '98-'99 he played 22 minutes per game, so he was a real contributor to both teams.