Jeffe Kennedy, this one's for you (not because she asked, but because someone asked HER). I've had this post brewing for a while. It took me a while to find all of these websites, so I think it's a worthwhile resource. Plus, sending someone a link to this webpage is a hair more polite than sending them the Let Me Google That For You link.
Bent on Books (Jenny Bent)
BookEnds Literary Agency (primarily Jessica Faust)
Between the Lines (Books & Such Literary Agency)
Caren Johnson Literary Agency
Crowe's Nest (Sara Crowe)
Dystel & Goderich Literary Management
Et in arcaedia, ego. (Jennifer Jackson)
Janet Reid (of FinePrint Literary Management)
KT Literary (Kate Schafer Testerman/Daphne Unfeasible)*
Upstart Crow Literary
Waxman Literary Agency
Writing and Rambling (Nephele Tempest @ The Knight Agency)
Wylie-Merrick Literary Agency
The Book Deal (Alan Rinzler)
The Swivet (Colleen Lindsay, former agent, currently at Penguin)
Note that not all these agents or editors are going to be for you. At the most obvious level, some only represent children's literature, or Christian literature, or some other genres and categories that you don't write. Many of these blogs will be very helpful for you to read anyway, but then please pay attention to submission guidelines when you are sending out queries, so you don't end up wasting everyone's time.
**** ETA: as of 11/6/10, Nathan is no longer an agent, but the archives of his blog are a goldmine. Just like Miss Snark.
Update: GalleyCat now has a list of the Best Literary Agents on Twitter for your added convenience.