J.J. Putz is underrated by the average baseball fan. Probably not by fantasy players, as it's hard not to notice he's 20/20 this year with converting saves. Also, I'd point out his 1.11 ERA, .65 WHIP, .140 BA against, and 36 K's to 6 walks. I would listen seriously to someone arguing that he's the best closer in baseball.
The "double play twins" (as a Mariners commercial portrays them with a strange hint of homoerotic flavor), Jose Lopez and Yuniesky Betencourt, definitely fly under the radar. Neither of them have put together any long stretch of great numbers, but Lopez was an All-Star last year, and even though Yuni is having some defensive troubles he's got a really special defensive talent, and is quietly hovering around .300. I'm not saying they're vastly underrated, but to the average fan they barely register.