MariaDB 5.5 is the current 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 10.0 is the development version of MariaDB. It is built on the MariaDB 5.5 series with backported features from MySQL 5.6 and entirely new features not found anywhere else.
See "What is MariaDB 10.0?" for an overview.
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.