FWIW, in addition to MariaDB, there is Percona Server as well, and they are also maintaining a launchpad for the base MySQL code on Launchpad while Oracle neglects that codebase.
* The ability to tune various parts of innodb that were originally designed to perform well on regular disks - those with fast sequential access, slower random access, and no real wear considerations. See http://www.percona.com/doc/percona-server/5.1/scalability/in...
I haven't looked at MariaDB 5.5 to see how it compares to Percona Server 5.5. I put Percona Server into production last fall and MariaDB was still at 5.3 at the time.
I've benchmarked all three and it seemed like the vast majority of the performance improvement on SSDs was due to the XtraDB engine which is included with MariaDB.
http://www.mysqlperformanceblog.com/2012/08/16/where-to-get-...