MySQL is one of the more interesting open source databases in that it allows you to select a different storage engine under the covers for each database. One of the most common and most robust storage engines (in that it supports transactions whereas some of the others do not), is InnoDB.
In a surprising move, Oracle purchased the company behind InnoDB and with it the InnoDB storage engine. What this means for existing users of InnoDB at this point is anyone’s guess, although Oracle are thought to most likely to keep the database engine open source. If they don’t and they use it as the basis for their own proprietary products, there will surely be a group that will maintain the open source code line for InnoDB outside of Oracle so that the open source version will continue to live on.