MySQL AB makes almost all of its money from the following sources:

• Training and Certification
• Support
• Consulting
• Licensing under very specific circumstances

And, from what I understand, the licensing revenue is a pretty small part of the pie. Not surprisingly, the first three are the same ways that RT, Shadowcat, and Stonehenge makes their money. The Postgres Foundation makes money from donations. I suspect many of the organizations that the PG devs live in support themselves in similar ways as the companies previously mentioned. I know that this is the case for both MySQL and Perl and Ruby.