Here are the links that I've found most useful in solving some of the more common issues in css.

1) Some CSS Tips/Tricks that may do not know, but should:
Mezzoblue CSS Crib Sheet
Webcredible's 10 CSS tricks you may not know
Webcredible's 10 more CSS tricks you may not know
Evolt's Ten CSS tricks you may not know

2) How to get all the browsers to have the same margins and padding:
Kurafire's Starting with CSS: revisited

3) How to deal with IE's faulty box model (The first 2 show you what the difference is between margin and padding):
Brain Jar's CSS Positioning
Red Melon's Interactive CSS Box Model Demo
The many box model hacks: CSS Discuss: Box Model Hacks

4) How to fix various float/margin bugs in IE (including the doubled margin bug)
Position is Everything's Floats, Margins and IE

5) Dealing with margin collapse issue:
Research Kitchen's CSS: Auto-height and margin-collapsing
Andy Budd's No Margin for Error

6) IE6 Peekaboo bug:
Position is Everything's IE6 Peekaboo Bug

7) Disappearing background:
Position is Everything's The IE/Win Disappearing List-Background Bug

8) General: Using Floats"
Max Design's Floatutorial

9) General: Making menus from lists
Max Design's Listamatic

10) General: Centering content with css
Simple Bits' CSS Centering 101

Other helpful web resources:
http://www.w3schools.com/css/
http://www.css-discuss.org/

Barrie North from Compass Designs also strongly recommends Ingo Chao's On having layout

Getting your head wrapped around the "hasLayout" in Internet Explorer can make to easier to understand many of the IE bugs and how to solve them.