Another missing category
Pliant is implementing a completely different database engine.
The general idea is to store the database as a tree, which
is much more flexible that the 'tables' model, and is mapping
staight forward the URL notion. With Pliant database engine, a database is stored in a single file even if it's a complex database.
The second idea is that the database is handled in the main memory, and the disk file format is HTML like ASCII with each modification appent immediately at the end of the file so that killing the database server process won't hurt.
See Pliant project at
for extra details.