Janus Software has an open-source SQL database product which can be run in what they call “Oracle mode”, essentially (according to Janus) replacing Oracle. The database product is called Firebird; the Oracle-mode version is called Fyracle. Since it’s open source, it’s free, or at least has no licensing costs, and apparently is supported on Linux, Solaris, HP-UX, and Windows at least. They even claim to be PL/SQL compatible:
I won’t be suggesting this as a replacement for my Oracle production databases anytime soon, but it might be worth looking at for some applications.