Oracle provides a number of useful links on OTN for people wishing to get started with the Database.

Notably the database homepage with links to download the Oracle Database, which may be freely used for self-education, and the online and downloadable Oracle Database documentation set.If you have a valid license for Oracle you can also download the software from Edelivery.oracle.com for production purposes.

The latest version of the Oracle database is 11g (homepage on this wiki here), released in August 2007 for the Linux 32bit platform and in October 2007 for Windows 32bit and Linux 64bit, now available also for Solaris, AIX and HP-UX. This is scheduled to become available for other Operating Systems by the end of 2007. The latest version that has been fully released is 10gR2 (homepage on this wiki here).

Release 9iR2 (homepage on this wiki here) is the oldest supported version (patchset 9.2.0.8), though older versions (Oracle 8i, Oracle8 and Oracle7) are still being actively used. Starting with 9.2.0.8, Oracle offers
Lifetime support

Oracle's full documentation set can be found at Tahiti.oracle.com
Additional notes, techniques and tricks can be added here.