SQLite is a lightweight open source SQL database library which can be integrated into standalone applications. It is supported by a Python library and would be an appropriate tool to use on the OLPC in applications which need a relational database. Unlike more relational databases, SQLite does not require a separate server application.
Two Python interfaces
And a higher-level Python interface
The OLPC has recently moved to Python 2.5 as the platform on which Sugar is built. This version includes SQLite as one of the standard libraries.