Below are various resources I’ve used that are particulary helpful
JavaBlackBelt.com – Java certification site, this is a fantasic site that allows you to take community produced exams on various areas of Java development (plus some other areas such as SCM, .NET). You first create a profile, start as a white belt, and as you progress through the exams you rank up and earn reputation. The site is very community driven and you are expected to contribute with questions as you gain ranking. Very good for brushing up on some skills or prepartion for certifications
StackOverflow.com – One of my favourite sites, I visit many times per day. This is a technical Q&A website where you can ask technical questions, related to programming. People can reply answers and can be voted up/down based on relevance to your original question. The site is community driven, but as reputation is at stake you are often guaranteed to get better answers than you would on a forum, since people can get something out of helping.
JavaRanch / CodeRanch.com – Probably the best source of Java forums there is. These forums were setup and managed by Bert Bates and Kathy Sierra, the two authors of Head First Java and the SCJP certification book. Definately worth a bookmark and daily visits
Head First Java – I got this a while ago to brush up on some core functionality that I had skipped over when I first started out. Its a really good book and explains Java very well, there are plenty of quizzes and questions on the various topics covered, and is one of the standard books that Java developers start out with. My only criticism is that its a little too easy, but thats down to me, not the book ;)
SCJP Manual – Best book for studying for the certification. It can be quite in depth and a little heavy in places, but its the best resource for studying the SCJP
The Java Posse – Good all round Java podcast, can sometimes be particulary focused on a specific area which may or may not be in your field, but its good to keep your horizons open isnt it?