There are tons of "Best Resources for (fill in topic)" articles out there, so I don’t expect this to be incredibly ground breaking. However I think it’s important to have a solid list of resources. I’m going to start with CSS. I attempted to make this list quality. I wanted CSS resources for all levels of CSS knowledge. There will be plenty of more resource articles in the near future, so this is just the beginning! Some of these sites you may know, others you may not. Regardless, there is tons of info to be gleemed from all of these sites. Enjoy!
A very comprehensive guide to all things CSS. This site has resources for everyone from the first-day-beginner to a militant-almost-to-the-point-of-violence anti-IE CSS expert.
Not to in-depth or detailed, but a good quick CSS cheat sheet. This would probably be best suited for beginners, or people with bad memories!
Web Developer is a FireFox extension that will help you a great, great deal when coding CSS. (and a whole lot of other things) It allows you to see the CSS, edit the CSS live, and many, many other things. It’s a "must have," no matter how cliche that phrase is.
Smashing Magazine is another all around great design resource. Most people have stumbled upon, or somehow came across a Smashing Magazine post. Guess what, they also have a CSS section. And it’s pretty flippin’ good. They have tutorials and articles, from beginner level to advanced.
This is a really cool tutorial that takes you through creating a CSS layout from scratch. It is meant for the beginner, with only basic knowledge of how to use CSS.
This is a really cool site that has about 50 blank, fixed width CSS layouts. This can save you alot of time. Also save you some headaches if you just would like a standard compliant start to your design.
This is a really good post about CSS typography. It’s basically a cheat sheet for all of the different things you can do with type with CSS. Speckyboy.com is also just in general a good resource, as they have alot of CSS/Web Development related articles.
This is a tool that you can use to create different menus with CSS. It can help you create a really cool horizontal, vertical, or breadcrumb menu with little to no CSS knowledge. Can also be helpful to cut the time it takes to code your idea.
One website I’m sure the majority of people are familiar with. A-List Apart is a GREAT web development resource covering all topics of web design and coding. This link is specifically for there articles about CSS, but check out the laundry list of other stuff they have to offer.
This is a really good article entitled "CSS From the Ground Up." And it’s exactly what it sounds like. It walks you through CSS from beginning to end. Obviously better for beginners, but it has a wealth of info coders from all walks can pick something up from.
CSSVault offers inspiration, and resources. They have a pretty cool gallery of sites you can browse, as well as an area for resources on all things web design. Pretty good amount of info/tips/inspiration can be picked up here.
This is a really, really cool website. It’s amazing what can be achieved just with CSS. On this site, people submit redesigns of the site, editing ONLY the CSS, leaving the source HTML files the same. It is amazing what some of these people can come up with. It is very inspirational, so make sure you check it out often.