I'm going nuts coming up with new and exciting command-lines to interrogate the common feed store using my new PowerShell provider. Check this one out:

cd feed:
dir -rec |
   where {$_.Author.Length -gt 0} |
   group Author |
   sort Count -desc |
   select -first 20 Name, Count |
   ft -au

That'll get you the top 20 authors across your entire feed list. Note that this is not looking for the feeds with the most items - it's the authors, so if one author posts to more than one feed then he's counted in both places. That said, in my feeds at work Robert Scoble is winning by a mile:

 Name Count
 ---- -----
 Robert Scoble 156
 Zonk 63
 mabster 60
 Michael S. Kaplan 53
 daveburke 52
 Jason Haley 44
 Cyrus Farivar 44
 kdawson 42
 crucible 42
 Mitch Denny 40
 Leon Bambrick 36
 Paul Miller 33
 bsimser 32
 GregLow 31
 ieblog 31
 oldnewthing 31
 Long Zheng 31
 Darren Murph 30
 Carl Franklin 29
 Daniel Moth 28