Cassandra is moving to a monthly release process called Tick-Tock. Even-numbered releases (e.g. 3.2) contain new features; odd-numbered releases (e.g. 3.3) contain bug fixes only. If a critical bug is found, a patch will be released against the most recent bug fix release. Read more about tick-tock here.
The latest tick-tock release is 3.5, released on 2016-04-13.
Apache Cassandra 3.0.x is supported until May 2017. The latest release is 3.0.5, released on 2016-04-11.
Apache Cassandra 2.2 is supported until November 2016. The latest release is 2.2.6, released on 2016-04-26.
Apache Cassandra 2.1 is supported until November 2016 with critical fixes only. The latest release is 2.1.14, released on 2016-04-26.
Older (unsupported) versions of Cassandra are archived here.
New users to Cassandra should be sure to check out the Getting Started guide.
The Cassandra source code is available to download:
git clone http://git-wip-us.apache.org/repos/asf/cassandra.git
Bleeding edge development snapshots of Cassandra are available from Jenkins continuous integration: trunk