ROMA User-Customizable NoSQL Database in Ruby
From the presentation:
- User-customizable NoSQL database in Ruby
- Features
- Key-value model
- High scalability
- High availability
- Fault-tolerance
- Better throughput
- And…
- To meet application-specific needs, ROMA provides
- Plug-in architecture
- Domain specific language (DSL) for Plug-in
- ROMA enables meeting the above need in Rakuten Travel
The ROMA source code: http://github.com/roma/roma/
Reportedly has 70 million users and while that may not be “web scale,” it may scale enough to meet your needs. 
Of particular interest are the DSL capabilities. See slides 31-33. Declaring your own commands. Something for other projects to consider.