The latest stable release of Apache Cassandra is 1.2.5 (released on 2013-05-19). If you're just starting out, download this one.Apache provides binary tarballs and Debian packages:
Starting with version 0.8, Cassandra supports CQL (Cassandra Query Language), an SQL-alike alternative to the traditional RPC interface. Drivers are available for a variety of languages and can be downloaded from their respective Apache Extras hosted pages.
The next major version of the Cassandra Query Language, CQL3, is scheduled for release in Cassandra 1.2, but an evaluation version of CQL3 is optionally available in Cassandra 1.1 (use cqlsh -3 to access it).
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 1.1 branch is 1.1.11 (released on 2013-04-19).
The lastest release on the 1.0 branch is 1.0.12 (released on 2012-10-04).
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.