I've heavily used different Internet messaging in both the nonprofit and the for profit realms. Being able to quickly chat with developers during a project is a powerful tool. Skype is great because it not only allows you to IM but to talk and to transfer documents securely. I've also used ICQ and AIM. I still use iChat (on the AIM network) regularly as well.

IRC started it all though and is still actively used by a wide variety of different groups. IRC is cool because there are long standing communities who discuss and support one another on a vast swath of topics. IRC is cool because they were Web 2.0 WAY before Web 2.0 had ever been conceived.

If you want to browse lists of communities, there is a good index at searchirc.com.

If you don't have an IRC client, Firefox has some plugins available. Personally, I use Colloquy, a super Mac IRC client. You can look at a list of other clients on ircreviews.org.