I wanted to be able to tab over some text in my posts, and spacing them over wasnt working (i hate that about html). Here is the code I used to add it.
Added in bbcode.cs at line 69(just after "r_bullet":
static private Regex r_tab = new Regex(@"\[tab]", m_options);
Added in bbcode.cs at line 172(just before "// bullets" ) :
// tabs
bbcode = r_tab.Replace(bbcode, " ");
You can change the width of the tab if you want by adding or removing one or more "nbsp" from the code.