I was browsing around Digg.com and found this excellent compilation of 71 CSS Menus. Highly recommended.
CSS Poster is a free online tool from chami.com. Just upload a CSS file, and CSS Poster will make a chart based on your CSS file.
Here is a cropped example using Drupal's Bluemarine template's CSS file:

A potential use for CSS Poster that immediately comes to mind is using it as a reference when customizing Drupal themes, or any kind of CSS file that was created by someone else.
If you want to preview what fonts will look like with different CSS rules, try fonttester.com. You can change the CSS rules in the left sidebar, and see the fonts updated in real time on the other part of the screen. When you find something you like, it will generate the CSS code for you. It's a excellent tool, especially if you are just learning CSS and want real-time feedback to your CSS changes.

Making a custom Drupal theme is actually quite easy. A Drupal theme is just a few PHP files and a CSS file. I prefer the PHPtemplate theme engine (the default one) but you have several choices. See the bottom of this post for a link to the official Drupal Theme Developer's Guide which has information on other Drupal template engines.
The following information was originally written for Drupal 4.7, but the concepts also work for Drupal 5 and 6 too.
I found an interesting list of web design links at devlisting.com.
Also check out the excellent reference page, the web developer's handbook, and the accompanying blog.
There are many good CSS editors for Linux. Basically, all you really need is a text editor that will perform syntax highlighting on CSS files. Some of the programs below are just simple text editors, while some offer additional features.
There is a developers extension for Firefox called DevBoi that gives easy access to documentation for Ruby on Rails, Ruby, PHP, CSS, JavaScript and HTML, among others. I first discovered it through the blog at quotedprintable.com.
Here is a screenshot of browsing Ruby on Rails classes in Firefox with the DevBoi extension:

HTML is what you need to know to make websites. XHTML is a newer form of HTML. CSS is a way to apply design elements to your HTML or XHTML such as color, fonts, borders, margins, etc. Start with the first link below: W3schools.com.
W3schools.com -- the best place to start to learn website design. Introductions to HTML, XHTML, CSS, and much more.
HTML Dog -- A site that teaches beginning HTML and CSS.
HTML / XHTML Templates -- a basic frame for your document from webstandards.org.
w3.org -- The World Wide Web Consortium. HTML and CSS specifications.
HTML Entities -- There are certain characters that can't be entered into your web site content directly because the browser won't display them. For example, what if you want to display the following on your web site: <p>? If you actually type those characters into your HTML, the browser will think you are starting a new paragraph and won't display the <p>. To have the browser display <p> you have to enter "<p>". The "<" is the entity representing the left angle brackets and the "> is the entity representing the right angle brackets. You can also use HTML entities to display characters not found on the keyboard such as √, ∴, ç, and €.