Web Technologies

First of all, Microsoft Development Center contains information about most web technologies. I particularly use Microsoft's site for JavaScript specifications.
http://msdn.microsoft.com/library/default.asp

The World Wide Web Consortium develops standards and guidelines for web technologies (HTML, JavaScript and much more). It is the place to pick up specifications and they also have some nice validation tools. Keep up with the development.
http://www.w3c.org/

If you want to dig into more advanced use of JavaScript and XML (also known as AJAX), I am currently in the process of reading Michael Mahemoff's Ajax Design Patterns book that thoroughly describes a multitude of different ways to achieve dynamic websites.

To keep track of upcoming Internet Explorer features, I often read up on the blog of the IE development team.
http://blogs.msdn.com/ie/

Usability of websites can be crucial to the user's perception of the company or web application. Although usability isn't really my favorite topic, I frequently read Jacob Nielsen's Alertbox weblog because it just is important!
http://www.useit.com/alertbox/

Many of the web technologies allow you to look other people over the shoulder all around the web to get ideas and inspiration (but don't forget that you are not allowed to just copy other people's work). I once made a JavaScript calculator which is still available here.

But if you really want to dig deeper into the web technologies, I recommend that you buy a book. I have personally been very glad that I (many years ago) bought JavaScript Unleashed from SAMS because it serves both as a tutorial the first time you read it and a reference later. If you are going to study a topic more in-depth, it is certainly nicer to have a book that you read in your favourite chair rather than reading a lot on-screen. The only caveat is that many web technology books have extraordinarily many pages and most of the time, the extra pages are just filled with repetitions and shows that the author has not managed to pick out the central points of the topic that he is to convey.