If you are looking for a conversation textbook that addresses lower-level learners, then you want one (or all) of the Survival English series. There are 3 books in total and they are aimed at situations that adult learners would find themselves in or would want to be able to discuss. The level is basic but there is plenty of review to build confidence.
Cambridge offers a number of ESL resources, including those that focus on speaking. For weaker intermediate students, their title "Lets Talk" is ideal, as it has reading and listening practice in addition to conversation. The self-study section for students allows them to review at home with their CD for further pronunciation practice. Stronger students can use "You Said It," which follows a storyline to help them develop conversation skills as well as more stylized structures such as speeches.
"Present Yourself" and "Great Ideas" are two excellent conversation textbooks for students who need less practice with grammar and more practice with expressing themselves and improving their fluency. Both draw on a variety of topics from movies to topical world issues such as global warming.
If you have higher-level learners who want to practice their language in an intercultural context, or who enjoy discussing intercultural issues, then try Intercultural Language Activities. The book comes with a CD-ROM with printable worksheets for the activities, and provides a wealth of material to talk about from cultural differences at home to work.