OSM has traditionaly run nearly everything off on one box. The wiki, mailing lists, source repository… everything. These little bits have now been moved to a VM machine donated by bytemark. Thus the wiki is now at wiki.openstreetmap.org, subversion source is at svn.openstreetmap.org, mailing lists are at lists.openstreetmap.org and the bug database trac is at trac.openstreetmap.org.
All this takes load off of the main machine and allows for easy migration in the future.