I was thinking about it this morning and decided to break it out via a chart. This is a rough number, and in it I give Percona points for the Xtrabackup.
Most of the work that I see, which is not niche work, is done by Oracle, hands down, no question. I don't see any sign of Heikki being very involved anymore but his legacy seems to be alive (or at least embers of them see to have not gone out). Percona provides a lot of niche changes to Innodb, and the Percona XtraBackup Tool (which if you don't know about, you should).
I, and others, look at changes that occur to Innodb for Drizzle. We adopt ones we are comfortable with(keep in mind, that I personally look at MySQL, PostgreSQL, and a couple of other open source databases as well). I am well aware of Innodb's short comings (and even more so with its connection to mysql's monolithic kernel, I noticed that in 5.5 they changed the default engine to Innodb, but they didn't change the testing frame work to use it for all tests).
HailDB is just offering testing right now for the embedded technology. This is giving projects that used the embedded version of Innodb a home since Oracle seems to have shutdown development on it (i.e. no new releases). HailDB is focused on getting the code in shape for open source development (and a better testing framework is being added to keep any regression issues from popping up).
MariaDB, i.e. Monty Program, is distributor of Innodb technoloy that they obtain from Percona.
I saw the note this morning about SkySQL opening its doors for doing business. They state "Through our relationships with strategic partners such as Monty Program AB", which would give them access to the old optimizer team from MySQL, but that is about it. Monty Program derives its knowledge of Innodb via Percona (and relies on their backup technology).
So in the end? At this point it appears that everyone is just a value added reseller of Oracle and Percona's work (though Percona is obviously a distributor of Oracle technology as well).
If you are interested in knowing about, and participating in the Ecosystem around MySQL you should either be attending or providing a talk at the O'Reilly MySQL Conference and Expo. This is the conference of the year for MySQL the technology, and I expect this years conference will be the best place to learn about what is happening next.
Update I was asked about bugs released to Innodb as the default engine. To get a good feel for it, force the tests in mysql-test to run as Innodb. We found very few bugs in Innodb itself (Innodb is relatively free of bugs), but the kernel was just not designed for it and it fits in more as an afterthought, not a design decision.
If you are up for another challenge, turn on Heap to just use its range index and not its hash index type.
Update2 I pulled a comment about replication that I had in the original post. Replication deserves more then a midstream one liner.