From Mario Fan Games Galaxy Wiki

BBCode (Bulletin Board Code) is a set of formatting tags used by MFGG's phpBB software, its previous Invision software and various other message boards. They can be used to apply various color and formatting styles to text, usually done to make posts more interesting or to stand out more.

BBCode Reference

Bold [b] [/b] Makes text bold Bold Text
Italic [i] [/i] Makes text italic Italic Text
Underline [u] [/u] Underlines text Underlined Text
Strikeout [s] [/s] Strikes out text Strikeout Text
Email [email] [/email] Links to an email address
URL [url] [/url] Displays a URL
Size [size='x'] [/size] Changes size of text. 'x' can range from -7 to 23, default is 2 Large Text
Font [font='face'] [/font] Changes font of text. 'face' must be the name of a font on the user's computer, but some fonts do not work Courier font
Color [color='color'] [/color] Changes color of text. 'color' must be the name of a color, such as "red", or its hexadecimal value, such as "#FF0000" Red Text
Image [img] [/img] Displays an image Red!box.gif
List [list='type'] [/list] Creates a list. 'type' can be omitted for a bulleted list, 'i' for a Roman numeral list, 'a' for and alphabetical list, or '1' for a numbered list. A [*] tag precedes each list item
  • Item 1
  • Item 2
Quote [quote='info'] [/quote] Displays quoted text. 'info' is optional QUOTE
Quoted text
Code [code] [/code] Displays code without formatting CODE
Code here
Superscript [sup] [/sup] Superscripts text Text superscript
Subscript [sub] [/sub] Subscripts text Text subscript
Spoilers [spoiler] [/spoiler] Hides spoilers, must click on Show Spoiler button to read Spoiler:
Spoilers here
Spoilers [ispoiler] [/ispoiler] Hides spoilers, must hover to read
Snape kills Dumbledore
Enable HTML [doHTML] [/doHTML] Allows usage of some HTML tags HTML-enabled text
Preformatted Text [pre] [/pre] Makes preformatted text
Youtube Video [video] [/video] Embeds a Youtube video
Youtube Audio [ayoutube] [/ayoutube] Embeds a Youtube video with a small height, usually used for posting songs on Youtube
Flash [flash=width,height] [/flash] Embeds a flash object with the specified size
Sizable Image [simg] [/simg] Displays an image that can be scaled up and down, and also links to the original image
Emoticon Displays an emoticon in the post without the need for [img] tags.

MFGG's main site also uses a watered-down version of BBCode. Some of the basic functions such as [url], [quote], and font ([b], [u], [i]) tags are built in.