Here's a helpful CSS cheatsheet!

[trevor]

Selectors

.class Selects all elements with .class
#id Selects all elements with #id
* Selects all elements
div Selects all <div> elements
div, p Selects all <div> and <p> elements
div > p Selects all <p> elements with a parent of <div>
div + p Selects all <p> elements placed immediately after <div> elements
div~ul Selects all <p> elements preceded by <div> elements
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Attributes
[attribute] Selects all elements with the specified attribute
[attribute=value] Selects all elements where the specified attribute is equal to ‘value’
[attribute~=value] Selects all elements with an attribute containing the word ‘value’
[attribute|=value] Selects all elements with an attribute list starting with ‘value’
[attribute^=value] Selects all elements with an attribute beginning with ‘value’
[attribute$=value] Selects all elements with an attribute ending with ‘value’
[attribute*=value] Selects all elements with an attribute containing the substring ‘value’

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Pseudo-classes
a:link Selects all unvisited links
a:visitedSelects all visited links
a:hover Selects links on mouse hover
a:active Selects the active link element
p::after Insert content after <p> element
p::before Insert content before <p> element
input:checked Selects every checked <input> element
input:disabled Selects every disabled <input> element
p:empty Selects every <p> element with no children
input:enabled Selects every enabled <input> element
p:first-child Selects every <p> element that is the first child of its parent
p::first-letter Selects the first letter of every <p> element
p::first-line Selects the first line of every <p> element
p:first-of-type Selects every <p> element that is the first <p> element of its parent
input:focus Selects the <input> element which has focus
input:in-range Selects <input> elements with a value within a specified range
input:invalid Selects all <input> elements with an invalid value
p:lang(language) Selects all <p> elements with a lang attribute equal to ‘language’
p:last-child Selects every <p> element which is the last child of its parent
p:last-of-type Selects every <p> element which is the last <p> element of its parent
:not(p) Selects every element that is not a <p>
p:nth-child(2) Selects every <p> element that is the second child of its parent
p:nth-last-child(2) Selects every <p> element that is the second child of its parent, counting from the last child
p:nth-last-of-type(2) Selects every <p> element that is the second <p> element of its parent, counting from the last child
p:nth-of-type(2) Selects every <p> element that is the second <p> element of its parent
p:only-of-type Selects every <p> element that is the only <p> element of its parent
p:only-child Selects every <p> element that is the only child of its parent
input:optional Selects <input> elements with no ‘required’ attribute
input:out-of-range Selects <input> elements with a value outside a specified range
input:read-only Selects <input> elements with the ‘readonly’ attribute specified
input:read-write Selects <input> elements with the ‘readonly’ attribute not specified
input:required Selects <input> elements with the ‘required’ attribute specified
:root Selects the documents root element
::selection Selects the portion of an element that is selected by a user
#id:target Selects the current active #id element
input:valid Selects all <input> elements with a valid value

Hopes this helps you guys!

[julian]

Nice list @trevor ! I personally use this list, provided by the w3c (not to be confused with w3schools, lol)

[trevor]

@julian Believe it or not, but I've never went there before for CSS reference, but I'll read up on it further here since of course they are the "authority" in web standards.