[zedadex]

For me it was Google Talk. It hit the sweet spot of a lightweight chat app that didn't take any configuration or convincing for my non-techie friends. I still miss it.

[benoliver999]

And you could use XMPP.

[kybernetikos]

There were audio bridges across to POTS. I used it to talk to people on other chat systems. It was great and felt like the future. Then it was gone.