There are currently two active releases available:
The latest release of Apache Cassandra is 2.1.3 (released on 2015-02-17). If you're just starting out and not yet in production, download this one.
The most stable release of Apache Cassandra is 2.0.13 (released on 2015-03-16). If you are in production or planning to be soon, download this one.
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.
The lastest release on the 1.2 branch is 1.2.19 (released on 2014-09-18).
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