Annually, I purchase the Writer's Market Guides for Literary Agents and Novel/short Story. Also, I have a 2005 copy of Jeff Herman's Guide to Publishers, etc. I generally check with AAR http://www.aar-online.org/database.html. I will very likely look into the Literary Market Place http://www.literarymarketplace.com/lmp/us/index_us.asp
Being in N.C., I checked into the N.C. Writer's Network, but their listing of publishers is not extensive (maybe 30).
[This message has been edited by kathmandau (edited December 21, 2004).]
As far as I know, only four major publishing imprints currently accept unsolicited manuscripts: TOR, Baen, DAW, and Ace. If you know of any more, please let me know. The others are usually open to queries, except Time-Warner imprints, who aren't. Small press is trickier: basically you must decide for yourself whether they're worth it. Did I mention you should avoid anyone who charges fees?
I run a small list of literary agents who accept speculative fiction here. I vet 'em as best I can, but the above cautions still apply, with the addendium that you should mail me if you find anything odd. *grin*
Good luck!