I've always thought MySQL was open source and free. Has that changed or can you still use it for free?

Depends on how you use it.

MySQL is released under the GPL, which means that if you bundle it with a non GPL application (e.g. for a commercial project) it is not free. You must purchase a commercial license then.

This is not limited to "bundling" the MySQL binaries (e.g. embedded usage), but as far as I know this already qualifies if you just add e.g. the connectors. But I'm not 100% on that. The exception in the license for e.g. the JDBC driver is only understable for a lawyer.

If you just "use" it then I believe it's free.

I always thought that. An ISV that I know of has an application that can use MySQL as a database, or SQL Server. They claim that they have been getting billed from MySQL and for that reason are pushing the free version of SQL Server.

I always wondered if that might be the secret business model of MySQL

The restrictive license and the technical deficiencies of MySQL make me choose PostgreSQL over MySQL any time. Or SQL Server that is (especially with the limits that the free version has)

Thanks for the comments!