As reported by Steven Feuerstein in his PL/SQL programming newsletter this month, it is now possible to compile and run PL/SQL applications in IBM's DB2 database! Check out this demonstration video and more details at IBM's Developerworks.
Of course, the list of supported built-in packages is rather short (for example, UTL_HTTP and the XML packages are missing), but they do have support for REF CURSORS, CONNECT BY, and other advanced Oracle features.
This is cool, because it opens up a new market for all us PL/SQL developers out here.
Worth mentioning, too, is that EnterpriseDB (based on PostgreSQL) also has PL/SQL support.