Percona provides a patched version of MySQL with many enhancements. I believe they incorporation many of the useful community patches, such as from Google, and some of their own to make InnoDB function better on SSDs, etc (at least that was the case in the past, many of those may have made it into mainline MySQL now). They also provide their own storage engine, XtraDB, with it's own set of interesting features.

Additionally, they provide a set of utilities as the percona toolkit to automate more advanced things, such as replication testing through table data checksums, zero-downtime table changes, etc.

Most of it is free, I think some advanced XtraDB features require payment (?), but they sell support and training.