Even good classroom management can break down at times students are transitioning: from one class to another, between activities, or at the end of a class. “Bell-ringer” activities take advantage of these times. They are not unrelated busywork. They are brief—usually written—activities that encourage students to focus their attention or reflect on the lesson.
At the beginning of the class, have the bell-ringer (or warm-up) ready when the students come in. They should get started right away, even before the bell actually rings. The students are engaged while you take attendance, distribute or return assignments, or check homework. Some examples of warm-ups include:

Answer a question about yesterday’s work or another related topic;
Respond to a statement or visual to uncover any misconceptions or to activate prior knowledge of the topic;
Solve a quick brain-teaser or math problem;
Complete a vocabulary entry with a graphic organizer such as a Frayer diagram;
Do a “quick write” with several sentences on a theme or topic;
Do a “quick draw” on a theme or topic;
Put a few words on the board and ask the students to write a sentence using all of them; and
Respond to a “this date in science history” or current event

At the end of the class, use another bell-ringer (sometimes called an exit activity or a ticket-out-the-door) as a formative assessment to check student understanding through a summary or a brief response to a question. This also gives time to scan the room to make sure materials are put away. Beyond classroom management, exit activities get students to focus and reflect, instead of dashing from the end of one class to another without “packing up” their thinking. But be sure that the exit activity doesn’t make students late for their next class. I know a teacher who has an official “time keeper” in the class to give him a five-minute warning!
Bell-ringers have many formats. Some teachers use a notebook page. Others use a single sheet of paper divided into sections for each day or small pieces of paper (recycling old handouts) that can be turned in. If students have laptops, they can add to a class blog or Wiki. Do students respond positively? They may not at first, but don’t give up! It may take time for this to become a routine.
Some teachers grade bell-ringers; others include them in a class participation rubric. Some collect them and then return them at the end of the unit to review. But be sure to skim them to identify what students do or do not understand. Refer to their work the next day: I noticed that yesterday some of you had questions about… It seems like you understand… I saw an interesting connection between… I observed a teacher who asked the students to put a “Q” in the top right corner if they wrote a question or a check mark if they wrote about what they learned. He skimmed through the papers and used the questions and understandings to guide the next lesson.
You asked, “Do they really work?” I haven’t seen any formal published research on bell-ringers per se (a possible thesis idea for graduate students?). But research (described in Robert Marzano’s Classroom Instruction That Works ) shows positive effects for strategies often used in bell-ringers (e.g., activating prior knowledge, the use of nonlinguistic representations, and summarizing). I’ve seen them used in all kinds of schools, grade levels, and subject areas. My own action research sold me on the topic, and I wouldn’t teach a class or conduct a workshop without bell-ringers.
If anyone has examples of bell-ringers activities you’d like to share, please add a comment.