Discord is a free-to-use VoIP and instant messaging app targeted primarily at gaming communities.
Available directly in the browser (requires plug-in), or as a Chrome app, or as a mobile app for Android and iOS.
C3 is a completely free to use voice and text chat application made for gamers. It was created by Vivox, who have and continue to provide integrated voice services for some of the more popular AAA games on the market. It can be used for MAC and PC.
Skype is a very popular cross platform freemium voice-over-IP service and instant messaging client.