Oracle Technology Network: PL/SQL:Official product documentation, white papers, technical briefs, feature lists.
BNF Index of PL/SQL Version 2.1 for Oracle 7: Backus-Naur Notation for PL/SQL 2.1 for Oracle 7. Old version, but still interesting.
Code Samples and Suggestions: Over a dozen tips, tricks, and code samples, by Jonathan Lewis.
FAQ by Frank Naudé: Part of the so-called "underground" Oracle FAQ, this is a brief but useful list of PL/SQL questions & answers.
High Speed PLSQL: NOUG presentation on High Speed PLSQL Tuning by Rodger Lepinsky.
ITToolbox - PL/SQL code exchange: Public forum where programmers can submit, retrieve, and rate PL/SQL code fragments
Ixora Q & A: Questions with answers by Steve Adams, expert on Oracle internals
Message board on orafaq.net: Unmoderated questions and answers.
mod_owa: Apache PL/SQL Gateway Module: Open source Apache module allowing PL/SQL to generate dynamic web pages.
mod_plsql: An Apache module. It aims to provide Oracle Web Application Server 3.0 API compatible interface to Oracle RDBMS. Tested with Oracle8/8i.
Oracle Larry Bot: An artifical intelligent agent who can be queried for Oracle Error Messages. He is currently accessible via AOL IM.
OracleTricks - Tips, Tricks, and Utilities: Free, reusable PL/SQL scripts; page is part of larger collection of Oracle tricks.
PL/SQL tips: Short tips and code fragments written (or collected) by Connor McDonald. Includes bit operations, undocumented features, and 8.1.7.
PLNet.org: An open source, focused, common repository for PL/SQL developers. Intended to improve the day-to-day life of thousands of Oracle developers around the world.
RevealNet PL/SQL Tips: PL/SQL Tip of the Month and archives.
Selected Utilities/Frequently Asked Questions: Scripts and solutions from Oracle maven Thomas Kyte, including a PL/SQL generator to address the "update cascade" problem.
UC Davis - Tutorials: Oracle SQL and PL/SQL tutorials from the UC Davis Database and Information Systems Group (DBIS)
utPLSQL Project: Open-source unit testing framework based on principles of "Extreme Programming."
Wikipedia: PL/SQL: Encyclopedia article about the Oracle extension to SQL, including an overview of its functionality, data types, and constructs.