Bold:
[b]bold[/b]

Italics:
[i]italics[/i]

Underline:
[u]underline[/u]

URL:
[url]http://wordpress.org/[/url]
[url=http://wordpress.org/]WordPress[/url]

Image:
[img]http://s.wordpress.org/style/images/codeispoetry.png[/img]
[img=Code is Poetry]http://s.wordpress.org/style/images/codeispoetry.png[/img]

Youtube:
[lyte id="YOUTUBE_ID_HERE" /]

Quote:
[quote]Lorem ipsum dolor sit amet, consectetuer adipiscing elit,[/quote]
[quote=NAME]Lorem ipsum dolor sit amet[/quote]
[quote="NAME"]Lorem ipsum dolor sit amet[/quote]
[quote author=NAME]Lorem ipsum dolor sit amet[/quote]
[quote author="NAME"]Lorem ipsum dolor sit amet[/quote]

Color:
[color="red"]named red[/color]
[color="ff0000"]hex red[/color]
[color=#ff0000]hex red again[/color]

Strikeout:
[s]striked this out[/s]

Center Text:
[center]center me[/center]

Computer code:
[code]function HelloWorld($greet = 'World') { return 'Hello '.$greet } [/code]

Font size:
[size=10]10px font size[/size]

Ordered lists:
[ol][li][/li][/ol]

Unordered lists:
[ul][li][/li][/ul]

List Item:
[li]item[/li]

Named Spoiler:
[spoiler=two plus two]four[/spoiler]

Unnamed Spoiler:
[spoiler]Boo![/spoiler]

Contents appear to logged in users only:
[user]This is a secret message[/user]

Contents appear to non-logged in users only:
[guest]Log in to see secret message...[/guest]