IF YOU ARE A CLIENT LIBRARY WRITER please update this list as your libraries become compatible with the newest version of cassandra.
High level clients
Using one of these clients is strongly preferred to raw Thrift when developing applications (the Thrift API is primarily intended for client developers). What follows are clients that support Cassandra 0.8.
Docs(Ton of em): https://github.com/deanhiller/playorm/wiki
Cassandrelle (Demoiselle Cassandra): http://demoiselle.sf.net/component/demoiselle-cassandra/
Feedly-Cassandra (ORM library): https://github.com/kireet/feedly-cassandra
Cassandra PHP Client Library: https://github.com/kallaspriit/Cassandra-PHP-Client-Library
If you are using Cassandra 0.7*, see ClientOptions07.
Thrift is the Cassandra driver-level interface that the clients above build on. You can use raw Thrift from just about any language, but it's not particularly idiomatic in any of them. Some examples are given in ThriftExamples.
The StorageProxy API is available to JVM-based clients, but unless you really know that you need it you should probably be using a higher-level client listed above or, failing that, the Thrift API. The StorageProxy API is intended for internal use, and highly specialized use-cases.
Running Hadoop map/reduce jobs in Cassandra is described in HadoopSupport.