EDIT: Do you know what secondhand embarrassment is? Go to an open mic and you are sure to experience it. I've seen some people who thought they were hilarious and were actually a train wreck.
Debate is another alternative. Besides speaking in front an audience it also teaches you how to articulate your thoughts and engage your listeners which is invaluable for presentations and the like.
Yes. You have to pay some attention to the rule set you use (if you are into formal debate). Some commonly used rules can have strange effects and encourage behaviour not useful in presentations.