I think almost everyone has heard of Skype for audio and video calls. What some people are less aware of is that the text chat functionality can be used for persistent chatrooms like those available through IRC and other chat protocols. Whether you've heard of IRC or not, you've surely heard of IM. IRC is a text based protocol that supports very large chatrooms. Anyone with a server can host multiple rooms and there are many highly active IRC servers still around. There is huge traffic on irc.freenode.net and I regularly discuss iOS/Android development with other developers there. The huge advantage over trawling and posting to bulletin boards is the real time feedback you can get from experts in the field. Of course the experience can be mixed and less hot topic related rooms, e.g. AI and P2P might lack anyone to answer your question, while other rooms, particularly on other less reputable servers, might well be filled with anarchistic script kiddies, but even then the conversations can be hilarious if not totally productive.
IRC can also be hopelessly addictive, and I carefully limit my exposure in order to get work done. Also, frustratingly, if you get logged off, or you sleep your computer, you miss any chat activity that occurred while you were away. Not so with Skype, which delivers the entire log of a chatroom to you when you log back in. This makes shared Skype chat great for my HPU Computer Science classes, and the HPU Computer Club chatroom I created seems to have really taken off. That's not to say everything is perfect. I'd love to see email notification integrated properly, since if someone types into a skype chat while you are logged off, and then logs of themself, you won't neccessarily receive the chat message until both you and the person who posted the message both log in together. Also Skype no longer provides public chats so you have to be invited in, although that has both pros and cons :-)
However one huge advantage of Skype is the great support for mobile, which is lacking in IRC. I have it set up so that whenever any of my students asks a question in chat my phone vibrates, which helps me give them instant feedback.
Anyhow, I've created a TechHui chatroom, and if anyone is interested in joining just hook up with me on Skype (username "tansaku") and I'll add you in and show you around!