[phicou]

We (developers and sysadmins) use it at the big corporation where I work. Keeping irssi open in screen (GNU screen) means I can connect remotely from anywhere with a single session, and it doesn't appear when I'm focused on something else in a different screen session. Each project gets its own channel. It's way better for collaboration than having to match schedules for each little question.

It's also very helpful for asking specific questions about OSS projects, as mentioned. The people actually developing projects hang out in IRC, not at StackOverflow.