Here are some of the websites I have found most useful. If you have suggestions, let me know.
If you are looking for an explanation of a convention, try these sites:
I usually go here first: The Bridge Guys
Some authors whose books I have found useful also have websites with good information.
My all-time favorite is Eddie Kantar
I have also found Mike Lawrence's website to be chocked full of good stuff.
If you like bridge blogs, Bridge Blogging gives you one-stop shopping for a variety of bridge bloggers.
This free windows-version double-dummy analyzer from bridgify has been very useful in helping me analyze hands.
I wrote a series of articles for the Superiorland Duplicate Bridge Club newsletter. You can find a list of the topics and links to the articles here.
Here's information about my Bridge Book published by Master Point Press.