That's what phpBB2 says about BBCode: "BBCode is a special implementation of HTML. BBCode itself is similar in style to HTML, tags are enclosed in square braces [ and ] rather than < and > and it offers greater control over what and how something is displayed."
is a PostNuke 0.71+ Module that adds support for BBCode. This module can be used by other PostNuke modules to integrate BBCode.
Note: see PostGuestbook for an example