I put together the following diagram to explain what the origin of the current MySQL forks and deltas looks like:

But in the distributed revision control world we live in, it's never really that simple. Here are some other notes:

• XtraDB is an InnoDB fork and "mostly" only a storage engine. It has a future in MariaDB and Drizzle, but may not make it into MySQL (about to be owned by Oracle, who also owns InnoDB).
• OurDelta collects more patches than just the Percona patches, and enables the PBXT storage engine. There's also a repository of OurDelta for 5.1 - binaries just aren't out yet.
• Like XtraDB, PBXT has a future in Drizzle, MariaDB and *maybe* also MySQL (Oracle may be more friendly towards PBXT than XtraDB, but Oracle is not known to be friendly so who knows!). I left PBXT absent from the diagram for not having much of a lineage with previous MySQL releases (flames welcome if you disagree).
…