XMPP with PGP Encryption over TOR or I2P (other anon net access like VPN, ...)
(OTR encryption is also very common for XMPP)

there are some semi-dead encrypt. p2p messenger projects like:

- TorChat
- Crypto.cat
- RetroShare

which had/have some community/users in the past


Tor Project is developing a new Instant Messenger to be bundled with their Browser/TOR Client downloads (will be an hardened fork of Instantbird with OTR encryption, that only uses TOR obvious)

I guess this will be the "market leader" / anon messenger with the most users medium term.