After I explained why I don't recommend querying agencies, Maria asked: What about querying managers?
Beyond the fact that your letter may never get past an assistant who has no interest in helping you, I actually think you might have better luck querying managers. Why? Agents are looking for material that is polished and ready to be sold. Sure, many agents will give notes and such - but this isn't the primary function of an agent; an agent's primary function is to know the marketplace, have relationships with all the buyers and be able to sell their clients' work (or otherwise get them a job). And it's rare that someone sending a query letter is going to be at that stage. Some people have this sense that agents are only interested in the sale and not long-term career planning, which I don't think is true...good agents will take all of this stuff into consideration. But developing material and making it better isn't usually an agent's focus.
Managers, on the other hand, are more likely to sign writers who need a little work. They recognize talent and help these writers develop their material before it is ready to be taken out and sold (or used as a sample to get staffed or get a writing assignment). Managers generally have shorter client lists than agents, and spend more time on each client. Many managers also produce, which is why they might take an extra interest in the material itself.
I have heard agents say things like, "Well, if he has some talent, we can take him to one of the managers we like and tell the manager to come back to us when it's ready." Managers and agents are often allies in this way. Managers will bring us clients they think are ready to sign with an agency, and we bring them writers we think aren't quite there yet, but have potential.
Still, I maintain that cultivating personal relationships will always be more effective than querying - and like an agency, a management company is going to be more focused on its current clients or writers with a lot of heat than signing people off queries. But if you're intent on papering the town, you'd be silly to target just agents and not managers.
I hope they don't hate me for this...but management companies Circle of Confusion and Benderspink both accept queries.