For the development team I use Slack for "chat room" style conversations. It's very IRC-esque.
For the marketing team we use project management discussion software where everything is tied to issues and we use Google Hangouts for the chatting.
For everything else and at the dealerships we use Microsoft suite - Lync is the current implementation. It's good software and works on OS X as well, so they use it across all 6 dealerships at one automotive group and don't have any issues with it.
If you have Office 365 for your email, you get their corporate messenger Lync with it
You can also use Skype which is free but lets your people chat with their friends too
You can do the same with google chat if you are using google for email service at your dealership/group
It looks like the nice 'newer' platforms are probably a little too complex for some of the older people here to get into. Basically I need an old school style AIM/yahoo, but with a forced and universal 'buddylist' of strictly coworkers. Brosix seems like its going to work for that..