MariaDB 5.5 is the previous stable (GA) release of MariaDB. It is MariaDB 5.3 + MySQL 5.5.
See "What is MariaDB 5.5" for an overview of what is in MariaDB 5.5.
MariaDB 5.3 has subquery optimizations, join enhancements, group commit, dynamic columns, microsecond support and windows performance fixes. See "What is MariaDB 5.3" for an overview of what is in MariaDB 5.3. Windows, Linux, and Solaris builds and Debian and Ubuntu repositories are available for this series.