Speaking in pairs or in small groups is less threatening to begin with. You can adapt one simple activity to practice any topic, grammatical structure or vocabulary set. Write questions on cards, or ask students to supply the questions. Give each student one card and ask everybody to stand up. Each student asks another his question. It is best if you participate as well. When they have finished, they swap cards and go to talk to somebody else. This is also an excellent icebreaker on the first day of a course.
Role-play activities work well for people who are not confident about speaking a foreign language. They aren't being themselves, they are acting a part and this can reduce inhibitions. Organize role-plays based on the functions you've taught in class, ranging from simple ones, such as buying groceries, to more complex actions, such as making up an excuse for a boss. Students take turns to play the shop assistant and customer or employer and employee.
Simple board games with question cards get small groups speaking in a very non-threatening way. You can make your own games with appropriate, open-ended questions for the level such as "Describe your happiest moment." "Snakes and Ladders" is a basic layout to model your game on; draw a grid, add "question" squares as well as the snakes and ladders to make a board, make a copy for every four students, write out question cards and provide game pieces and dice.
Once your students have developed more confidence speaking, you can introduce activities that involve speaking in front of the whole class. Role-plays can be developed into short acts. Debates on issues that most people will have an opinion on, but are not too controversial, are another option. A simple, fast debate activity is to elicit the names of examples for several groups of items; for instance, cities, foods, animals and styles of music. Two students come to the front of the class, each chooses a different example of the same topic and makes the case for why their city, food, or animal is better. The class votes and the winner of the debate remains standing, to be joined by another student. They then move on to the next topic and so forth.