Oracle XE (Express Edition) is the cheapest (free) way to access to the most powerful RDBMS, it includes most of the features as those of a regular edition. Oracle XE is a subset of the Oracle SE and is pretty simple to install and manage.
Oracle XE can be downloaded from the regular download site at otn.oracle.com, and there are three editions, one for Windows (certified for Windows Vista too), one for Linux (red hat compatible) and one for Debian compatible.
It has been installed on several platforms, RHEL (4 and 5), OEL (4 and 5), Fedora, Debian, Ubuntu, White Box, Ubuntu, SLES (9 and 10), and others, considering the common kernel they have.
People working with XE doesn't have a support account at metalink, so the main source of support is the one they can get from forums, particularly at forums.oracle.com either on the general forum, the install forum or the speciallized XE forum, where the user must first register to have access to it.