PersistClass provides access to relational databases for PHP. It attempts to focus on simplicity and convenience by eliminating the need for boilerplate, meaningless code. It can establish database connections (MySQL, Oracle, etc.), execute SQL queries, parse query results, provide convenience methods (read a single row or value, etc.), escape data (against SQL injection, HTML escaping for outputs), handle transactions, and provide strict error handling and object persistence (storing and retrieving PHP objects in db tables).
Objectify-Appengine is a thin Java wrapper around Google App Engine's low-level datastore API that allows you to persist type-safe POJO objects. It provides a human-friendly query interface, generified key, query, and prepared query classes, GWT-able entity objects, and transactions in one lightweight jar file with no dependencies.
Tephlon is an easy PHP persistence library. It has a highly modular OOP design and a no-SQL storage system. It stores any object or variable safely and tidily on disk for fast access. The API is meant to be easy and fast to learn. It features Java-style key-value maps, namespaces, easy Codeigniter integration, programmable lifetime of records, a multi-driver architecture, reliable and well-tested code, and easy incremental backup of data with SVN or GIT.
minidb is a Python module that utilizes the SQLite3 database library in order to store and retrieve Python objects. It utilizes Python's __slots__ mechanism to determine the column names, and uses the class name for table names. Data is always stored as text in the database, but will be converted using the type specified in __slots__ (which therefore has to be a dict).