This is the best introduction to styles and stylesheets. Not long on examples but great for starting and for reference.
HTML Source : http://www.yourhtmlsource.com/stylesheets/index.html

Another nice introduction to web design with style sheets. This one's 4 pages in length.
http://www.glassdog.com/design-o-rama/webdesign-css.shtml?css

The Web Developer's Guide has an intro page at http://wdvl.internet.com/Authoring/Style/ or you can go to the brief tutorial which is worth looking at even if it's a somewhat naff design
http://wdvl.internet.com/Authoring/Style/Sheets/Tutorial.html

Stylesheets without tears is a short and concise intro to style sheets. Nice examples show you how to use margins, font size and line height in pixels, and set table colours and fonts for every row and cell
http://www.wpdfd.com/editorial/wpd0402.htm#feature

“CSS Layout Techniques: for fun and profit” is an excellent page. You'll find easy to follow instructions to build 2, 3, 4 column page designs and more.
http://glish.com/css/

The web designer's blog called “A List Apart” has some dead brill stylesheet HOWTOs.

• Practical CCS Layout - Tips, Tricks, Techniques. This gives a blow by blow account of how to use CSS instead of tables. We used the first simple technique in class, the “Splitting the Difference” section shows how to get a cool looking navigation bar. The section on “CSS Help” could also be useful.
  http://www.alistapart.com/stories/practicalcss/

• CSS Design - Taming Lists deals with the flexibility you can achieve with lists and CSS. Particularly impressive is the roll-over button look-alike list at the end. Can't wait to implement this one!
  http://www.alistapart.com/stories/taminglists/

• Other great articles on this site are:
  “Fear of Style Sheets” - and how it was conquored:
  http://www.alistapart.com/stories/fear/
  Redesign of site in CSS
  http://www.alistapart.com/stories/journey/All
  other articles on this site are worth a read such as the Dao of Web Design

• Also, Document Object Model (DOM) and what you can do with it:
  http://www.alistapart.com/stories/dom/index.html

Cutting edge design with style sheets. Check out the “complexspiral” link and salivate! This definitely works best in Netscape version 6.2 or 7 or Mozilla 1.
http://www.meyerweb.com/eric/css/edge/

Four short articles from Netscape Tools for Web developers — Consistent List Indentation, Defining Cross Browser Tooltips, Styling Abbreviations, Giving Cursor a Hand — which could be useful.
http://devedge.netscape.com/central/css/

A fairly technical overview of style sheets from a couple of years ago, so slightly dated. Entitled “CSS in the real world” it has sections, “What is it and who cares”, “Two approaches to CSS” , “Additional problems / solutions” , “Recommended Reading”
http://www.zeldman.com/lectures/css/

“/CSS/ - a guide for the unglued” is worth looking at if only for it's domain name — TheNoodleIncident — wonderful!
http://thenoodleincident.com/tutorials/css/

The Brain Jar has this article on Using Style Sheets. Goes into the workings in a lot more detail.
http://www.brainjar.com/css/using/

• Also from the brain jar an in-depth article about CSS positioning which explains the CSS “Box Model” in some depth
  http://www.brainjar.com/css/positioning/

Keep it Simple - tips on using fluid design with stylesheets. Even has a link to the Microsoft article about switching Doctype declarations (also below)
http://www.digital-web.com/columns/keepitsimple/keepitsimple_2002-09.shtml

CSS Shorthand - tips for expressing attributes in a shorthand way.
http://home.no.net/junjun/html/shorthand.html

ChunkySoup is another great domain name. This one has a great page on how to get over the tables to stylesheet hurdle called A Quick Guide To Making Style Sheets Work. Opinion features include Your CSS bores me with a fun design, and Most wanted browser features. Not your average Style-sheet site I am glad to say.
http://www.chunkysoup.net

Thoughtful article on why the author should redesign his site with CSS. Cool domain name too - acts of volition
http://www.actsofvolition.com/index.cfm?article=521

This is a great table of style sheet functionality. Worthwhile printing out
http://www.glassdog.com/design-o-rama/styles.html

A good reason to use Netscape 7 / Mozilla : Sidebar tabs for CCS2, HMTL 4, DOM 2
http://devedge.netscape.com/toolbox/sidebars/

Another good reference / tutorial site. The CSS examples are very useful and the Quiz is quite difficult!
http://www.w3schools.com/css/default.asp

Important article on CSS “enhancements” to Microsoft Internet Exploder 6. The most important bit is that IE 6 defines the CSS "width" for boxes (a term that gets used a lot when making CSS pages) in a different, non-standard way. Fortunately the fix is also here, and you can, and should coerce IE 6 to adopt standards compliant behaviour by including a !DOCTYPE declaration at the very top of the file ahead of the HTML tag. Here's an example of what you should include:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Strict//EN">

Webreview Stylesheet Reference guide. A bit dated, but has good sections on support of stylesheets by various browsers (but does not include IE 6 and Netscape 7).
http://www.webreview.com/style/index.shtml