The two MySQL forks with the most momentum seem to be MariaDB (from former MySQL owner Monty Widenius) and Percona (from the folk behind MySQL Performance Blog). Both have committed to be drop-in MySQL replacements, albeit with extra features. So, swapping to either one should need no changes in your code, off the shelf apps like Wordpress or the desktop tools that you may use to access MySQL.
While these forks exist, we have straightforward fallback positions if Oracle screw up MySQL. As such, I've decided that it isn't worth worrying about. If Oracle start doing things that actually affect my ability to use MySQL effectively, I may change that view and look at the alternatives more closely. As it stands, it looks to be more about disgruntled developers than a change of policy at Oracle.
Whether I'd want to commit to a MySQL fork longer term in an enterprise environment is a different story. If Oracle killed MySQL tomorrow, a fork would probably be a stepping stone to refactoring to use PostgreSQL.
