For Xbox:
http://www.xboxscene.com/ - Although emulation and such has come almost to a stop. There's occasionally activity on the forums.. the last big project that was being worked on was a port of AvP, but that started more then 2 years ago and has only had some beta releases.
 
For Dreamcast:
http://www.dreamcast-scene.com/
http://dcemulation.org/ - The old forums were wiped about a year ago if I remember right, so a lot of content was lost.
http://www.dcemu.co.uk/content/?s=c08f4dfeb36578efd83ee3a1fb44e5ef
http://dreamcast-talk.com/forum/news.php
http://dreamcast.es
http://www.assemblergames.com/forums/index.php
KOS http://cadcdev.sourceforge.net/
Linux http://www.lxdream.org/news/
NETBSD http://www.netbsd.org/ports/dreamcast/

I can't find the link to the Russian group who did Counter-Strike DC so here are two links about it:
http://www.assemblergames.com/forums/showthread.php?t=17052
http://the-dreamcast-junkyard.blogspot.com/2009/03/counter-strike-dc-final-10.html