Recommended starting points
Introduction to Cassandra at FOSDEM 2010 (video + slides)
Cassandra by Example, an explanation of the Twissandra data model and code. A demo is up at http://twissandra.com/. Ports to Java and C# are also available.
Maxim Grinev's posts on modeling basics, translating SQL concepts to Cassandra, and update idempotency
Recommended advanced material
Facebook's LADIS 2009 whitepaper (heavy on Cassandra internals)
Case studies
How Mollum replaced SSDs and an external cache with Cassandra
Reddit's switch from memcachedb to Cassandra. There are more comments by the Reddit engineer who did the work, ketralnis, in the reddit discussion thread.
SimpleGEO's use of cassandra, an interview with O'Reilly, a blog post with some infrastructure details, and slides from engineer Mike Malone
4 Months with Cassandra, a love story by Cloudkick
Looking to the Future with Cassandra: how Digg migrated their friends+diggs data set to Cassandra from mysql
Facebook engineering notes on implementing inbox search with Cassandra
Example how to model your data into nosql with cassandra - a case study on a facebook chat style application.
Other Articles
Migrating Cassandra between Data Centers, January 2012
10 steps to upgrade a Cassandra node, March 2011
Cassandra: the importance of system clocks, avoiding OOM and how to escape (unnecessary) OOM meltdown, February 2011
Installing, monitoring, and performance testing Cassandra on EC2
Cassandra Overview, Setup and Java Thrift Client : Part1, Part2 and Part3, June 2010
Up and running quickly in Java using Pelops, June 2010
A Cassandra Glossary (simple explanation of key terms)
Locking and transactions over Cassandra using Cages, May 2010
Cassandra Jump Start For The Windows Developer, March 2010
Cassandra: fact vs fiction: in which some misconceptions are corrected, April 2010
Embedding Cassandra in your JavaEE app server, April 2010
Installing Cassandra + Thrift on FreeBSD for PHP development
Creating a TimeUUID in .NET, April 2010
Installing and using Cassandra with Java, part 1 (setup), 2 (data model) and 3 (data model 2). Part 4 is titled Thrift Client but it's also another data model example, with code samples
Cassandra in action: round up of Cassandra articles and deployments in February and March of 2010, March 2010
Cassandra Internals: Writing and reading, March 2010
Haskell & Cassandra: First Steps, March 2010
Running Multiple Cassandra Nodes on a Single Host, March 2010
How To Configure Cassandra On A CentOS Media Temple DV Server and another CentOS installation guide, March 2010
WTF is a SuperColumn? an Introduction to the Cassandra Data Model. See also an independent Java implementation of the example model discussed here.
Hector: a Java Cassandra client, February 2010
Cassandra: RandomPartitioner vs OrderPreservingPartitioner, February 2010
Running Cassandra as an embedded service, February 2010
Benchmarking Cloud Serving Systems with YCSB: Yahoo's paper comparing Cassandra, HBase, PNUTS, and sharded MySQL performance.
Cassandra -- Getting Started: Cassandra data model from a Java perspective
Cassandra and Thrift on OS X: one, two, three, November 2009
Building Scalable Databases: Denormalization, the NoSQL Movement and Digg, September 2009
Meet Scalandra: Scala wrapper for Cassandra, August 2009
Cassandra and Ruby: A Love Affair? - Engine Yard's walk-through of the Cassandra gem, 2009
Up and Running with Cassandra: ruby-centric examples of a Twitter clone and a multi-user blog
Building a Small Cassandra Cluster for Testing, September 2009
Persistência nas Nuvens com NoSQL, Brazilian Portuguese, June 2011
Cassandra y Hector, Spanish, October 2011
Presentations
Introduction à Cassandra (in french) - Nicolas Romanetti, January 2012
Cassandra pour les (ch'tis) Développeurs Java - Jérémy Sevellec, December 2011
Cassandra Data Modeling Workshop - Cassandra SF, Matthew F. Dennis, July 2011
Cassandra/Hadoop Integration - Jeremy Hanna, January 2011
Using Cassandra with your Web Application - Tom Melendez, Oct 2010
CassandraとHBaseの比較をして入門するNoSQL by Shusuke Shiina (Sep 2010 Japanese)
On Rails with Apache Cassandra - Stu Hood, April 2010
Introduction to NoSQL and Cassandra at Outbrain, April 2010
Cassandra NoSQL EU April 2010
Cassandra NoSQL in Spanish
Scaling Twitter with Cassandra from Twitter's Chirp conference, April 2010
Outside the Box with Apache Cassandra from POSSCON, April 2010
Cassandra overview at the San Antonio JUG, April 2010
Cassandra at Frugal Mechanic from the Seattle Tech Startups Meetup (March 2010)
Introduction to Cassandra from the Austin Java Users Group (Jan 2010)
Cassandra for Rails from the Spanish Rails Conference 2009
Cassandra in Production at Digg from NoSQL East 09
What Every Developer Should Know About Database Scalability: presentation on RDBMS vs. Dynamo, BigTable, and Cassandra
NOSQL meetup video - NOSQL Slides: More on Cassandra internals from Avinash Lakshman.
Cassandra presentation at sigmod: mostly the same slides as above
NoSQL: onde, como e por quê? Cassandra e MongoDB: Rodrigo Hjort (Nov 2011 pt_BR)
Podcasts
The Cassandra Project - Technometria: Phil Windley interviews Jonathan Ellis on Cassandra. Date: 2010-03-09.
Dude, Where’s My Database?! - Techzing! #8: Justin & Jason talk to Jonathan Ellis about Cassandra. Date: 2009-07-01