Do you use these on your forum? Did you make your own or download them from somewhere?
My forum uses them, and I found all the icons from here: http://findicons.com/, originally I tried to use only circle shaped icons but they are all different shapes now. I think they make the categories and forums look more interesting.
I have some custom ones for my forum, but I would like for them to change when there are unread posts and back to read posts. If anyone can help me add them and let them do this, just PM me.
I can see the use for them and think some forums use them well - this forum for example. I don't use them myself though, don't know why I just haven't ever bothered with them before. It might be a good move to use them and I probably would if I got enough demand for them.
I love forum icons, I use IconArchive to get all my icons, it has the best to find them. I haven't really used FindIcons, I never had a good experience with them, not really sure why.
Most of my board icons however are on FontAwesome now so it took away the ability to really have board icons. 🙁