Tox is an open source instant messaging protocol supporting text, sending files, video chats and so on.
qTox, µTox, Antox, Toxic, Antidote and others are open source clients for this protocol. Available on Linux, Windows, Mac OS, Android, iOS and anywhere you ready to port it https://tox.chat/ Unlike Skype and ICQ Tox using decentralized DHT network for p2p communication. User ID (hash) looks like 0ECA261D206B8D3899A8B0B751BF37206E3CCD73ACE2C4C3B79A9CB9505B301C642D14F43AD7
Tox FAQ: https://wiki.tox.chat/users/faq
Here's a list of Tox bootstrap nodes for DHT network: https://nodes.tox.chat/ There's also so called ToxDNS for shorter and human readable contact names. It is possible to use 3rd party DNS or to establish own one: https://wiki.tox.chat/users/toxdns (it is considered to be less safe than hash, however if you control own DNS it should not be a problem)
Link to the Tox Core repo: https://github.com/irungentoo/toxcore (crossplatform C)