Configurable
Disk
Scalaris
On one end of the spectrum is Scalaris which is entirely memory-driven, and Redis which is primarily memory oriented (you can do background snapshots). Cassandra, BigTable, Hypertable, Hbase allow configuring how large the Memtable can get, so that provides a lot of control. The document stores - CouchDb, MongoDb and Riak …
Last week I got around to wrapping up my old Gravatar solution into an actual configurable Ruby on Rails plugin and hosting it over on GitHub .
Here is some example usage and configuration examples.
Install as you would any other plugin ./script/plugin install git://github.com/mdeering/gravatar_image_tag.git
Basic usage gravatar_image_tag('spam@spam.com'.gsub('spam', 'mdeering'), :alt => 'Michael Deering')
Configuration Points