Hacxx on Hacking Forums - My notes on RSS....
phpBB
/feed.php
/feed.php?f=4 - subforum in front of feed.php
vBulletin
/external.php?type=rss2&lastpost=1
http://www.forum.com/external.php?type=RSS2&forumids=20&lastpost=1
XenForo
/forums/-/index.rss
MyBB
/syndication.php?limit=50
/syndication.php?fid=8&limit=50
IP.Board
IPS
/discover/all.xml/
SMF
/index.php?action=.xml
Discourse CMS
/c/category.rss