For current stable release (2.8.5) check the documentation page.
Snapshots of the latest development version of Akka docs are available
Documentation: Java Manual Java API Scala Manual Scala API
Akka 2.6 was the previous stable version. Only patched if critical bugs are found.
Documentation: Java Manual Java API Scala Manual Scala API
Artifacts are available from Maven Central.
Akka 2.5 reached end-of-life 11/6/2020, with the last version being 2.5.32
Documentation: Java Manual Java API Scala Manual Scala API
Artifacts are available from Maven Central.
Akka 2.4 reached end-of-life 12/31/2017, with the last version being 2.4.20
Documentation: Java Manual Java API Scala Manual Scala API
Artifacts are available from Maven Central.
Akka 2.3 reached end-of-life April, 2017 with the last version being 2.3.16.
Documentation: Java Manual Java API Scala Manual Scala API
Artifacts are available from Maven Central.
Even older release are available from Maven Central for their respective version number.