I'm currently teaching large groups (50 or so) of college students, and I've had a lot of luck with roleplays. I pre-teach some of the grammar patterns they will need (making requests, etc.) and we brainstorm some content ideas (a list of requests one might make in a hotel, etc.) and then I give them about 10 minutes to do a roleplay with their partner.
Afterwards, I ask for volunteers to show the roleplay to the class and I write down grammar / vocabulary points that I think are relevant. For me so far, it's worked better than group discussions. I'm interested in trying some of the games mentioned here, though, especially the one Eric suggested about groups writing the questions for answers given by the teacher.
Take care,
Dave
ESL etc. - Bringing global issues and activism into language teaching.