Today 
an article in the Times recounts the use of classroom clickers to engage students in large lectures.  I particularly liked how students could foil their use as automatic attendance checks:  just give it to your friend to bring in!  But kidding aside, they seem like a useful tool to increase participation and learning.  I gave Twitter a go this fall and it didn't take, probably because I didn't set up anything specific for students to use it for other than their own questions.