Todd Hoff's highscalability.com has a lot of great coverage of NoSQL, including some case studies.
The commercial Vertica columnar DBMS might suit your purposes (even though it supports SQL): it's very fast compared with traditional relational DBMSs for analytics queries. See Stonebraker, et al.'s recent CACM paper contrasting Vertica with map-reduce.
Update: And Twitter's selected Cassandra over several others, including HBase, Voldemort, MongoDB, MemcacheDB, Redis, and HyperTable.
Update 2: Rick Cattell has just published a comparison of several NoSQL systems in High Performance Data Stores. And highscalability.com's take on Rick's paper is here.
