I got to try this out at a little mini-conf Twilio organized some months ago. It is, if anything, even more fun than "Hey you can reimplement Skype in 3 lines of code", because one of the parties to the call can be a computer. You can use the client as a "programmable audio pipe" direct to the user's browser.

We all got to do little weekend project demos. I made a website for learning the Japanese numbers where the computer would audibly quiz you, drill to kill, etc.

You can also make more intuitive interfaces for those underrepresented demographics in the world by having the computer programmatically narrate what they need to be doing on your website. (If you haven't tried this before, I recommend it: it works even better than the standard popup-based product tour.) Say you're eBay. Somebody is trying to list a antique China doll and can't seem to find the submit button. Why don't you ask if she would like you to walk her through it? If people aren't going to read on the Internet don't force them to read.

Or those lovely little "Heya I see you are browsing our website why don't you chat with me" applets. How's about making that vocal? "It's OK ma'am, just talk at your computer. You're looking for the buy button? Try the purple one on the top left of your screen. Yes, we do offer free shipping. You have a good day now."

You can also do the usual Twilio magic for business process automation. "Oh look, we can provision an entire call center with everything they need to do inbound or outbound work for the cost of headphones. No gazillion-dollar VoIP contract or phone lines required."