The March '08 issue of The Writer magazine has the lowdown on 25 agents currently looking for submissions. They cover what the agents want to see, what type of proposal to send, commission and complete contact information. Some of the listings offer response times, too. Writer's House, which has me as a client, is one of the agencies listed. If you're looking for representation, might be worth picking up the issue.
Reading listings like these can offer some interesting bits of info, too. Agents can be very specific about what they don't want, i.e. "queries only" and "no science fiction." They also drop helpful or encouraging hints, like "Especially interested in writers from the Pacific Northwest, the West, Alaska and the Pacific Rim" and "Unpublished writers considered."
Nine of the agents/agencies listed in The Writer also mentioned how many queries and manuscripts they on an annual basis:
Abrams Artists Agency: 1,000
Ethan Ellenberg Literary Agency: 10,000
Jeff Herman Literary Agency: 5,000
Linda Konner Literary Agency: 1,500
Nancy Love Literary Agency: 2,000
Jean V. Naggar Literary Agency: 6,000
Alison Picard: 5,000
Wendy Schmalz Agency: 4,000
Scott Treimel: 2,000
Take your calculator and divide those numbers by 250 (the average number of weekday workdays in a year) and you'll get an idea of just how many queries and manuscripts these agents receive on a daily basis. Now imagine reading them and responding to them. Yeah, I think I'll stick to being a writer.
I recommend checking out any agent thoroughly before you submit or sign on with them. Make sure they're a member of AAR, and try to get some recommendations from other writers you know on who they like, who they've had trouble with, and who might be a good match for you.
If you'd like more info on agents, Laura Resnick has an excellent three-part article series that covers agents, agent-hunting, and working with and without an agent, which you can read online here.