Another way to step out of your comfort zone is extemporaneous speaking. Toastmasters is an organization with meetings all over. There is a "table topics" section in each meeting that lets people give 1-2 minute responses to questions posed on the spot. Their Competent Communication program is great for learning to write and deliver speeches.
I'm always a little wary of self-help topics like this. Particularly if they have titles like "The Power of Yes."
But I want to mention that extemporaneous speaking is, certainly, a remarkable confidence-builder.
There is one greater, however: walking up to a complete stranger and striking up a conversation. Not "so, we're both waiting in this bus terminal, where are you going?", but engage them in a serious subject they have opinions on. Politics or religion, for example. These are very hard subjects to get people to talk about at any length.
Ideally, you will have a goal in mind -- to find out what interests them, to interest them in something, to change their mind about something, even just to find out their name.
Public speaking is theory, but conversation is application. Speaking TO people is secondary, speaking WITH people primary, and changing someone's mind the greatest mark of your ability to communicate.
"Start small. Many people are afraid to make phone calls, or to approach a clerk in a store. Begin by practicing these little habits. A clerk in a book store answers hundreds of questions a month. Theres no reason to be frightened of asking yours."
I used to be afraid of stuff like this. Then I started hanging out with new people, some of whom are even worse than I was. It's really helped. I ask them why they don't just do something. They confess that they're afraid. I ridicule them, knowing that I would also be too afraid in their place. Then I have to do it myself to demonstrate that it's not hard or scary at all.
Unfortunately I haven't yet applied the general rule.
It's not just "yes," it's "yes and!" Accept opportunities, but not simply as they're suggested. You plus the work, you look for the opportunity within the opportunity, you create something on top of what you're given, and you find a way to make it work better for you.
Find a club near you, yes?
http://www.toastmasters.org/find/