At the moment, I'm not using anything, but I have used gotomeeting and webex - both with teleconferencing - in the past. Some people I know have convened with SecondLife, but that takes a mammoth learning curve and a powerful computer.
But IMHO opinion, one of the technologies for quick, off-the-cuff meetings is the chat function, which is carried by many social programs, including the one that M.D. uses. If you look at the bottom right of the Firefox browser, you should see how many members are on M.D. Clicking on "Meetings Diva Chat" will bring up the chat browser.
In Chat, you can have public or private chats. One thing, hoever, you should raise the level of the alert message so you can hear when someone's hailing you. Also, I'd arrange beforehand a time when you'd be chatting with your cohorts - not unlike what you'd do in f2f meetings.