Surprisingly, the team with the best bullpen ERA right now is the Detroit Tigers with 2.22. The Tigers were 22nd last year, with a 4.34 ERA. They made some good moves in the offseason, although they probably didn't think they would turn out this well.
One of the biggest changes, though, came from someone who has been on the team for a long time, and has made huge strides forward this season.
Jose Valverde (signed as a free agent): 12 G, 11 IP, 0.82 ERA, 7 SV
Joel Zumaya: 10 G, 14.2 IP, 1.23 ERA
Phil Coke (Acquired from the Yankees in the Granderson trade): 12 G, 13 IP, 1.38 ERA
Eddie Bonine: 9 G, 12 IP, 1.50 ERA
Fu-Te Ni: 7 G, 10.2 IP, 1.69 ERA
Ryan Perry 9 G, 8.2 IP, 3.12 ERA
Their ERAs could change dramatically, of course, with one bad inning, but it seems like they have a solid group in the bullpen for the first time in a while. In their 2006 World Series run, they had the top overall ERA in baseball, and were 4th in relievers' ERA. In 2007, their bullpen ranking dropped to 23rd, and it was 27th in 2008.
It's probably no coincidence that 2006 was the last year where Joel Zumaya pitched in more than 30 games. It was also the last season where he had an ERA under 3.47 (it was 1.94 over 62 games in 2006). A couple of preposterous off-field injuries cost him a lot of time on the field, but he seems to have finally made his way back
A big part of his problem over the last few years has been his control. Last year, he gave up 34 hits and walked 22 in 31 innings, for a WHIP of 1.81. This season, though, he has allowed 12 hits and walked none in 14.2 innings, while striking out 16. Unless this is some kind of a bizarre early-season fluke, the Tigers will have one of the best 8th/9th innings combos in baseball with Zumaya and Valverde.
If he's healthy, there is no doubt that he'll strike out a lot of hitters (he throws over 100 mph), and he probably won't allow a lot hits. Even if he allows 2 or 3 BB/9, he'll be extremely tough this year. If he has Eckersley-like control all season? He could have one of the great seasons ever by a reliever.
They can probably count on good work out of Fu-Te Ni (2.61 in 36 G last year), Phil Coke and Ryan Perry (Eddie Bonine's 2009 stats aren't encouraging, but he does have great numbers so far this year).
No matter how badly Zumaya pitched or how many ridiculous injuries he sustained, it was always worth it for the Tigers to be patient, because he is a special talent. It's not everyone who can get Nolan Ryan to say they have the "best arm I've seen since I don't know when." Zumaya is one of the more interesting players to follow right now, because we have no idea how good he can be (who expected 16 SO/0 BB?), and he is clearly getting better.
It seems like he's been around forever, but he's only 25, so the sky is still the limit. The Tigers are probably still holding their breath that he won't hurt himself, but their playoff chances are looking a lot better now than it was before the season, and the bullpen is one of the biggest reasons.