Forum is an online discussion site where people form community and discuss/converse with each other through posted messages. You can reply to queries put by other in the community or ask a question. Being active in your community and giving advice and answer to others can build your reputation as an expert.
List of forum sites-
a) http://www.flickr.com/help/forum
b) http://www.addthis.com/forum
c) http://www.forums.cnet.com
d) http://forums.myspace.com/
e) http://forum.statcounter.com
f) http://www.bookforum.com
g) http://forum.videolan.org
h) http://ubuntuforums.org
i) http://forums.mozilla.or.kr
j) http://forums.seochat.com
k) http://forums.searchenginewatch.com
l) http://www.bloggerforum.com
m) http://forum.wordreference.com
n) http://www.city-data.com/forum
o) http://community.skype.com