Carlos Marmol Heath Bell Hideki Okajima Jonathan Broxton (minus recent blowup) Joba Chamberlain Santiago Casilla (when healthy) Joey Devine Dan Wheeler Scott Linebrink Cla Meredith
Honorable mentions (need greater sample size):
Joe Smith Leo Nunez John Grabow Taylor Buchholz Jesse Carlson Jorge Campillo
Kuo's a good one, though his role (long relief) is a bit different than the rest on this list. He really hasn't been pitching more than 1-2 times per week.
Raffy Perez should also be included I think. Although he started off poorly, he's been solid the past few weeks.
I'll throw Chris Perez of the St Louis Cardinals out there. Had 22 Ks in 17.2 IP before being called up a couple days ago. Also has a decent shot to be closing at some point this year...
Both Nunez and Ramirez from KC have had good seasons. Ramirez has 22Ks in 18IP.
Aqualino Lopez has had a quietly good season for DET this season.
Scott Downs and Jesse Carlson have both had good starts in TOR.
Octavio Dotel has had ridiculous K/9 in CHI this season (27Ks in 17.2IP). His ratios aren't as low as some of these guys, but that's mostly due to his first couple appearances. In his last 10 apps he's allowed only one earned run total.