The B&B concept—a handful of rooms run as an inn by a family in their own home—has spread throughout France, where it is called chambres d'hôtes (shaamb-doat).
Note there is a sub-category of B&B called chambres chez l'habitant ("rooms in the house of the inhabitant"), which operate bit more like a rental room than a full-service B&B (and cost a bit less).
Most Paris B&Bs charge about €50 to €120 ($65 to $155) for a double room.
For particularly nice or historic properties, prices might be in the €170 to €250 bracket ($220 to $325)—any time you see the words charme or charactre on a lodging in France, the price is going to go up.
A bed and breakfast or guesthouse essentially works something like a small hotel, one which provides breakfast and is located in the owner's home (or at least an a converted apartment in their building). In practice, this means a cozy, welcoming, friendly place and plenty of interaction with your hosts.
There are sometimes drawbacks to the B&B. With a hotel, you are guaranteed a certain degree of anonymity: you just ask for your key at the desk and then are left alone.
This is often the opposite of a B&B, where chitchat is considered part of le charme—but sometimes, you just don't want to make small-talk with the owners and other guests.
On the other hand, a B&B can be a great opportunity to meet some local folks and really get an inside scoop on the culture.
Also, at a B&B there's often a curfew, either stated or implied—after all, you wouldn't want to wake that kindly older couple up at 2am when you stagger back to your room, now would you?
This is the easiest and most direct way to pick precisely the B&B you want and book it immediately.
With an agency, you get a pretty wide selection and a guaranteed quality/level of service (if you use one of these agencies approved by the mayor's office). Some agencies also represent apartments and cultural experiences.
However, the properties listed tend not to be as cheap—there is an agency fee, after all—and the info on individual B&Bs can be a bit sparse beyond a brief description, location, and price range.
Oddly, most agencies will not let you book a specific B&B directly from the Web site. You have to fill out an e-mail form with all your details, dates, and requirements and they get back to you with a list of available B&Bs. (What is this, the 1990s?)
I therefore list these roughly in order of how useful they are: