To start, take a look at ASWB.org. They administer the exam. They also publish detailed content outlines for each exam (LCSW, LMSW...). If what you're studying isn't on the outline, you shouldn't be studying it!
To learn about what all that stuff on the content outlines you've never heard of is, use the device you're reading this on right now. Go to Wikipedia, go to About.com. For diagnostic info, try the Mayo Clinic site or Psych Central. Remember, you don't really need to know every detail about every item on that content outline. The exam tends to focus on the information that everyday social workers face every day. Focus your studying in that direction too.
Essential to any social work exam prep is reading and rereading the NASW Code of Ethics. The vast majority of exam questions are rooted in the code. Learn it!
Finally, it's time to try out some practice questions. Here's a collection of links to free practice tests. Included there, a fresh set of ethics questions on the SWTP blog, one question per ethics code section. Getting a sense of how all that content is shaped into exam questions is really helpful come exam time.
Done with all that? You haven't spent a penny and you're loaded up with great exam info. For some, that's preparation enough. But with all the money you've saved, you may as well try out complete practice tests to get a sense of what the four-hour sit is like. That'll cost some. But still you'll have saved a huge amount compared to what people used to spend to get ready for the test. Congratulations!
And congratulations in advance on getting licensed!