If you have fond memories of misspending entirely too much of your youth on web-based message boards, or you would like to misspend more of your nights and weekends on them now, Google has you covered. Simply add
inurl:viewtopic.php
to any query, and most, if not all, of your results will be discussion threads on boards running atop phpBB. Similarly, using
inurl:"index.php?topic="
seems to get Simple Machines fora, and
inurl:showthread.php
will unearth vBulletin fora. You can also combine them, like so:
croissant AND (inurl:viewtopic.php OR inurl:showthread.php OR inurl:"index.php?topic=")
Happy holidays, and good hunting.