NO SQL is basically a highly scalable disruptive data storage technology. The basic downsides included proprietary APIs (no standard SQL), evolving capabilities, loads of vendors, lack of skills.
Here’s some information from the net…
FoundationDB – has the added advantage of providing data consistency.
MapR - SQL capabilities over large-scale distributed systems including Hadoop and NoSQL databases
GridGain - brings in-memory capabilities to MongoDB. Achieves elastic scale and automatic transparent re-sharding
Scientel - Gensonix® stores structured/unstructured data in Relational, Hierarchical, Network, and Column formats, and scales to trillions of real-time transactions.
Accumulo - enable online model building and dynamic indexing to support both retrospective analysis and enrichment of streaming data.
Microsoft - Windows Azure Tables offer the best of both scalability and ACID guarantees.
RavenDB - a schema-less document database that offers fully ACID transactions, fast and flexible search, replication, sharding, and a simple RESTful API
eXist-db - High-performance native XML database engine and all-in-one solution for application building.
Cloudant - providing strong-consistency for single-document operations.
Aerospike - optimized for SSDs through a highly parallelized, distributed architecture.
StarCounter - an in-memory database that processes millions of database transactions per second on a single machine.