Some useful links for Java technologies:
http://java.sun.com - The official Java developer website - new articles posted weekly.
http://www.jguru.com - A great source for Q&A style interaction in the community
http://www.javaranch.com - Everything about Java and more. you can also get help for your problems in Java and prepare for certification. Best forum.
http://java.sun.com/docs/books/tutorial/ - The Official Java tutorial from Sun - very useful for almost any feature set. Must for detail study of Java.
http://www.javaworld.com - One of the originals. Weekly updates of Java articles.