The latest stable release of Apache Cassandra is 2.0.3 (released on 2013-11-25). If you're just starting out, download this one.Apache provides binary tarballs and Debian packages:
The current major version of the Cassandra Query Language is CQL version 3. The CQL3 documentation for the current version of Cassandra (2.0) is available here. The documentation for CQL3 in Cassandra 1.2 is here.
New users to Cassandra should be sure to check out the Getting Started guide.
Previous stable branches of Cassandra continue to see periodic maintenance for some time after a new major release is made. The lastest release on the 1.2 branch is 1.2.12 (released on 2013-11-25).
The lastest release on the 1.1 branch is 1.1.12 (released on 2013-05-27).
Older (unsupported) versions of Cassandra are archived here.
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.