Kristof Kovacs put together an excellent comparisons of the different NoSQL products. Here are the products it covers. MongoDB Riak CouchDB Redis HBase Neo4j Cassandra Membase
https://speakerdeck.com/u/okram/p/titan-the-rise-of-big-graph-data This presentation introduces Titan, which is a new graph database for bigdata. It integrates with a backend of either Cassandra or Hbase. Titan is created by Aurelius.
This is a lengthy but very good slide deck on the what/why of the tools used at Twitter. Note: The slide deck is about 2 years old. NoSQL at Twitter (NoSQL EU 2010) View more presentations from Kevin Weil