ANA. Sometimes the political views that ANA has and mine don't mesh but you get discounts on books and other things (like whirlpool appliances and continuing education), a magazine subscription...it is a little pricey.
What field are you in? There's pretty much an association for any specialty you have an interest in. I'd suggest you do a google search on whatever specialty you work in for nursing associations and take a look at their membership benefits on their website.
They often have discounts, scholarships for members, magazines and newsletters
If your state has an ANA chapter I recommend joining. That way you will find out about legislation in your state pertaining to your career and will be able to network with local nurses. If you have a speciality, like ER medicine, there are associations devoted to types of practice.
I think it depends on your specialty. For most people, the association of nurses in your specialty is the one that will have the most things that are most relevent to your work. Another thing to consider is whether or not there is a local chapter of that organization that is active. They might have local conferences, meetings, etc. that would provide you a way to network with other people in that same specialty -- making contacts that will be useful to you in the future.
Personally, I have always been a member of 3 or 4 organizations -- the ANA, Sigma Theta Tau, and one or two for my specialites.
You might want to reference a few web sites for your education, ongoing learning: CMA, Joint Commission, your state health department web site, your state board of nursing web site, PubMed (others continued below when I get the names).
Also, the FDA, Center for disease control, your state hospital association web site, Medscape, Nurseweek, Institute for Safe Medication Practice. I reference these on a regular basis to keep up to date on current rule changes, practice guidelines, etc.