"Oracle Database 10g Express Edition (Oracle Database XE) is an entry-level, small-footprint (150MB) 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."

Downloading it, however, involves a free (rather lengthy) registration process.

If you want to skip registration, here's the link:

The above link is to a Linux x86 rpm.