Oracle's Certification site - The place to start.
Oracle's Magazine - Oracle's monthly magazine
 
FREE Sample Tests

Cert21.com - online Oracle 8 and 8i practice exams, free registration
SelfTest Software - One of the best tests there is.  Download the free demo here.
Animated Learning - You need to fill out a form but they have some nice free online courses.
Oracles tests - Fill out the form and download these practice tests right from Oracle.
Docsoftware - 10 online test sample questions.

Study Materials

Crossword Puzzles - If like crossword puzzles and you want to test your Oracle knowledge, try this
Hot-Oracle - Some nice step by step lessons on various tasks.
Technet - Search Oracles online Technical database.
Oracles Downloads - Find Drivers, Tools and Utilities.
ORAFAQ - Very intensive FAQ.
WTAinc. - Nice selection of white papers.  Many of them are fairly outdated though.

Newsgroups and Mailing Lists 
comp.databases.oracle 
comp.databases.oracle.server 
comp.databases.oracle.tools 
comp.databases.oracle.misc 
comp.databases.oracle.marketplace 
Oracle Mailing Lists