Scot Shields has set the standard for the non-closing middle reliever in fantasy baseball - 80 to 100 innings pitched with 5+ Wins, a few saves, a strikeout per inning and great ratios. This type of performance is more valuable than a 200-inning #3 starting pitching on most teams. My favorite middle reliver is Jonathan Broxton. He is getting a lot of hype as the imminent Dodgers closer, but I'd be prefectly content with a repeat of last season's rookie campaign - 97 ks and a 2.59 ERA and 1.23 WHIP.
Another middle reliever who is also touted to be imminently closing is the Pirates' Matt Capps. He doesn't quite have the strikouts of the top relieving prospects, but he will throw in excess of 80 innings with a great WHIP. I rather have him than hope for a Carl Pavano comeback. Throw-in his proximity the the Pirates closing job, and I'm sold.
A third middle reliever who fits the bill is former minor league prospect of the year, Jon Rauch, of the Washington Nationals. From that lofty perch, he could never match expectations and has been forgotten by most people. However, he revived his career last season in the anonymity of the Nationals middle relief corps. With 91 innings, 85 Ks and solid ratios, he is the 3rd best fantasy pitcher on the Nationals staff behind Chad Cordero and John Patterson. With little hype and a more established closer in front of him, Rauch is likely to slip into the reserve rounds/$1 end-of-draft bin.