By far the best mod ever for phpBB can be found here:
http://ptifo.clanmckeen.com/
It not only gives you sub-category and sub-forum abilities, but it also increases the admin and user control and customization by ten!
Seriously, the creater of this mod really knows what he is doing. He has re-built the phpBB forum from the ground up, improving the code, adding features, streamlining and increasing the admin functions. The list goes on forever.
search-bot management
easy addition of acp and ucp fields
an authorization function to better control who can see/do what
Really, that's just the tip of the ice-berg. I am really, really impressed with this mod.
