We too use Hipchat, Skype and Hangouts. All for different things and since some tools offer features which others don't:

- Hipchat: Internal discussions and exchange, syntax highlighting, easy room and pwemission management, updates from other services (github, new relic etc). Furthermore to push notifications to email, mobile device and/or via text.

- Skype: easy 1on1 communication with externals (sometimes replaces the regular phone), internal LAN file transfer

- Google Hangouts: group video conferences (our company uses Google Apps so everyone has it and can access it from anywhere through a browser)

I probably missed some features and usecases, but I think I got the most important ones :)