Please give company/organization name, description of use case, and potentially a link to a blog post or press release for more information
Despegar - Despegar uses a Cassandra cluster to storage user sessions of its hotel booking site, and also as a persistent cache of flight itineraries.
Digg - The recent V4 relaunch is 100% Cassandra. We are running on multiple clusters internally, our largest one is 40 nodes spanning multiple datacenters. We also have 1 core commiter on staff.
Imagini - We use Cassandra to store all real-time data for our ad targeting technology (VisualDNA). We run a Cassandra cluster on Amazon EC2 nodes across multiple availability zones. We also use Hadoop for batch processing, integrated with Cassandra.
Ooyala - Ooyala uses Cassandra as the backing store for a near-realtime video analytics platform, which allows publishers to analyze and optimize the performance of their online video content. (See the press release.)
Rackspace - Rackspace uses Cassandra for a variety of internal needs. Several Rackspace employees are also core committers and contributors on the project.
reddit - Using Cassandra as a persistent cache and in the process of migrating other data-types from Postgres into Cassandra
SimpleGeo - We use Cassandra as our core datastore for providing location-based services and products. We run Cassandra in multiple availability zones within Amazon EC2. Some code can be found at http://github.com/simplegeo
Twitter - "We're using Cassandra in production for a bunch of things at Twitter." For details see this blog post from July, 2010.
Shazam - Shazam uses a small Cassandra cluster to power their recommendations system.
SoftwareProjects - Software Projects uses 20 Cassandra nodes across 3 datacenters to power the eCommerce platform for 3,000 businesses. We use Cassandra to store real-time purchase data and provide various stats for our customers. Cassandra is our primary data store.
eBay - eBay has Cassandra supporting multiple applications with rings spanning several data centers.
additional users may be found at Cassandra Users on DataStax.com