One of the tools we use most at longaccess is HipChat. Here is why.
HipChat is a chat service for companies. I'm not sure how it would work for large teams, but it's been of great value for our small team of four.
HipChat is our "back channel". We initially decided to use a chat service like HipChat in order to avoid the endless internal email threads and emails for trivial things like "I will be late today", but it has turned out to be much more.
We all have it open (on a browser or one of the native apps) all day and whenever we want to share something we post it there. Even when we are sitting at the office, one next to the other and we talk in person, HipChat is the easiest way to share the link we are talking about, or a short code snippet, or a command line or a file. And it obviously helps a lot when we are working from home, or travelling.
One other great thing with HipChat is it can be integrated with many other services. For example, we are heavy GitHub users. We have setup GitHub to automatically post to one of our HipChat rooms (you can have more than one, for example "General Discussion", "Developers", etc.) whenever a new commit is pushed to it.
This makes HipChat a great tool for everyone in the team to follow what's going on, even if some are not involved in a specific task.
If your team members are familiar with IRC, and you have the time and expertise to setup an IRC server and maintain it, you may want to go that way. But even then, the pricing is so low (free for teams of five or less, $2/user/month for larger teams), you may want to consider it as an option.
On the other hand, if you just want to focus on doing what your team is supposed to do and don't want to develop and maintain your own infrastructure, I would recommend to give HipChat a try.