Biggest turnaround this season?
Check out Joel Hanrahan.
In his first 34 games (and 33 innings), he posted 7.71 ERA.
In his second 32 games (and 30 innings), he's posted a 1.78 ERA.
Two things, mostly.
One, he was traded by the Nationals to the Pirates.
And two, he's gotten a lot, lot luckier.
As a National, Hanrahan struck out 2.5 times more hitters than he walked. As you know, that's a perfectly acceptable figure. He gave up one home run every 11 innings; also a perfectly acceptable figure.
So how could he possibly have a 7.71 ERA? Well, he gave up a whopping .431 batting average on balls in play (prior to this season, he'd given up a .306 BABiP -- a perfectly average figure).
As a Pirate, Hanrahan's strikeout-to-walk ratio hasn't been as good ... but he hasn't allowed a single home run and he's given up just a .311 BABiP.
Hanrahan's not nearly as awful as his ERA with the Nationals; he's not nearly as good as his ERA with the Pirates (though his ERA cannot be explained simply by his BABiP). He's just another hard-throwing relief pitcher who might be excellent if he could walk significantly fewer hitters (but probably never will). He's also an object lesson at looking beyond ERA if you want a good read on how well a pitcher has actually pitched. And this is particularly important with relievers.