There have been a number of sites lately, especially with the recent rise in endgame pickup groups, designed to help you find and examine potential PuGgers quickly. But none of them have made it quite as easy as Zarkmark, a site that allows you to quickly rate anybody you come across ingame by "zarking" (rating them up) or "marking" (rating them down) them. Then, they offer a quick search, which will not only give you a one-click link to the Armory, but an easy-to-read screen of how many people have rated the player you're looking at. Theoretically, ninjas will never find a group again.
Of course, theory is theory, and right now, the Zarkmark directory is pretty empty, so odds are that for any given player you look up, you won't really get much feedback. But you never know -- if players jump in and populate the site (and it wouldn't help to have a little help from them -- instead of just an Armory link, it would be nice to see a player's gear and achievements right there on the page), we might eventually get a pretty accurate picture of what someone's reputation looks like.
In fact, I'm a little surprised that Blizzard hasn't ever considered a reputation system in-game. Xbox Live carries one off pretty well -- even though I've never actually used it to consider who I do and don't play against, I have rated players and I know it's very easy to see scores if I wanted to. Given the rising numbers of VoA ninjas and PuGing in general, it might be worth it for them to give each player a socially-created rating in the LFG interface.