You could download and test Oracle's DBs for free for quite a while - but only now you can use them for free in a production environment. That's clearly aimed at MS' offerings like MSDE and SQL Server Express and not at MySQL or PostgreSQL.
Also Oracle is an enterprise DB and MySQL, PostgreSQL or even MS SQL Server can not be compared to it in that regard. This also means you need trained staff to administer it - forget about just downloading and using it. Tried to get a demo of Oracle's XML Publisher working - I know what I'm talking about
So if your shop is already using Oracle's DBs this is a nice offer for the occasional small project. But for everyone else, just stay with what you know and love - whether it's MySQL, PostgreSQL, MS SQL Server or something else.