You'll notice that a couple of times I refer to a service called Tiny Chat. While you could use Tiny Chat, I don't endorse it for classroom use because the Tiny Chat homepage at times displays content that would be inappropriate for a classroom.
Chatzy is a neat little website that I learned about from Wes Fryer. Chatzy provides a free platform for hosting your private chat area. To use it, simply name your chat area, select your privacy settings (you can password protect it), then send out invitations. Instead of sending out invitations you could just post the link to your chat area. Chatzy is a nice alternative to Tiny Chat because you can restrict access to it.
TodaysMeet is completely free to use. Setting up a chat area in TodaysMeet is very simple. To set up your chat area just select a name for your room (that name becomes the url for your chat area), how long you want your room to exist, and select an optional Twitter hashtag for your chat area. To invite people to your chat area send them the url. What makes TodaysMeet different from services like Tiny Chat is that TodaysMeet doesn't have a public gallery of chat rooms containing questionable content. TodaysMeet also doesn't place inappropriate advertising on your chat area.
Edmodo is a microblogging service designed specifically for educational use. Using Edmodo teachers can create a microblogging network for their classes. The latest version of Edmodo updates in real-time so that members of group can quickly respond to each other. Edmodo also provides teachers with a place to post assignment reminders, build an event calendar, and post messages to the group. To learn more about Edmodo watch the video below.
Present.ly provides a platform for creating your own private micro-blogging community. The free version of Present.ly lets you create a community based on your email domain. For example, if I had other people using freetech4teachers.com as their email domain, I could establish a Present.ly community just for people with that email domain. (Since I'm the only one with an at freetech4teachers email address, it would be a boring a community). The video below provides an introduction to Present.ly.
Although it could be difficult to get enough invites for all of your students to use it now, in the future Google Wave could be a great platform for back-channel discussions. Google Wave allows users to thread conversations, invite people into a conversation at any point, and see the text that others are typing as they're typing it. Wave also allows you to post links, embed maps, and a myriad of gadgets. Watch the video below for a concise introduction to Google Wave.