ABLE Dewey Tutorial
http://www.lili.org/forlibs/ce/able/course7/01index.htm
This online tutorial is based on the 13th edition of the Abridged Dewey Decimal Classification. Unfortunately, the tutorial refers to specific page numbers in the 13th edition -- page numbers that don't match the 14th ed. Even though the tutorial needs to be updated, it is still possible to follow along and get an introduction to the structure of the DDC and how to build numbers.

DeweyBrowser
http://deweyresearch.oclc.org/ddcbrowser/a14
Based on the 14th edition of the Abridged Dewey Decimal System. Does not include the tables, manual, or notes of the printed Abridged Dewey. A quick way to confirm a Dewey number or explore the Dewey Decimal System.

025.431: The Dewey Blog
http://ddc.typepad.com/025431/
Written by members of the Dewey editorial team at the Library of Congress. They often take a topic in the news and explain where it would be classed in Dewey and why. There are usually three or four posts a week. The Dewey team is not without a sense of humor.