If you want to start with developing free software on old technology, you can download the Oracle Database 10g Express Edition.
Actually, Oracle 10g is more than 5 years old and it is the only version available as Express Edition, 11g has not been released at the time of the writing, and no patch has been released, not even for the worst bugs or security breaches.
Most probably you will not want to run this for your sensitive data.
If you are serious about Oracle Development, you could download the Enterprise Edition for free, and get the Personal Edition on Oracle Shop for support and patches.
Last Friday 13th August, a news/rumor (not an official announcement) was posted on the opensolaris forum, apparently OpenSolaris has been cancelled in favor of an Oracle Solaris 11 Express. Not sure if it will come in the same format as the database XE, but this does not sound to please the community.
After the Oracle sues Google, it does not make Oracle very popular in the opensource community at the very moment