Java 5 and 6 enhancements

http://java.sun.com/j2se/1.5.0/docs/relnotes/features.html
http://java.sun.com/javase/6/webnotes/features.html

and a nice link to show diff between Java 5 and 6 beta
http://java.sun.com/javase/6/jcp/beta/apidiffs/index-diffs.html

Must reads on Threads and synchronization

Some of the excellent links that I have come across and which I wouldn't love to miss

Java Memory Model
Jermey Manson Blog
JMM from IBM Library
Volatile from IBM library
If I weee King: A proposal for fixing the Java programming language's threading problems
Writing multi threaded java apps
Multi threading