Oracle Database 10g Express Edition is free to develop, deploy, and distribute.
http://www.oracle.com/technology/products/database/xe/index.html
Oracle Database 10g Express Edition (Oracle Database XE) is an entry-level, small-footprint database based on the Oracle Database 10g Release 2 code base that's free to develop, deploy, and distribute; fast to download; and simple to administer.
Oracle Database XE can be installed on any size host machine with any number of CPUs, but this free version of the world's leading database will store up to 4GB of user data, use up to 1GB of memory, and use one CPU on the host machine.