I recommend the mod that I recently read about that will add a question or secret code word to your registration page. The mod is called
Registration Auth Code (rac.mod)
link to it is (www) phpbb.com/community/viewtopic.php?f=16&t=552845
I also recommend the admin only allowed posting html code mod. This mod is called
HTML For Adminslink to is (www) phpbb.com/community/viewtopic.php?f=17&t=491768
Another favorite mod is the one that manipulates the profile upon registration. When a user registers, the website and signature sections are removed, and they are not allowed to register if they enter in anything in that section (usually bots will do that!), this is handy for preventing a member registering soley to spam a website in their profile. After they've activated their account, they can then update their profile with whatever information they want. This mod is called
No Website or Signature during registrationlink to is (www) phpbb.com/community/viewtopic.php?f=15&t=373695
One of the most appealing mods to me was the attachment mod. In fact, it was the very first mod I had experience with in changing codes. It took me a great deal of time to figure out. The thread on the
Attachment mod is so huge over at phpbb site

I guess many people had troubles following instructions! I think it is a nice addition to your board.
link to is (www) phpbb.com/community/viewtopic.php?f=15&t=74505
There are lots of great mods. I just thought I would point out a few that I have found beneficial. There are more, but I will spare you the long details!
Have fun and take your time.....
I've never implemented any mods. Is it easy? Does it cost money? Are there any easy how to guides available for it?
Hi, they are pretty easy to install if you take your time and read the instructions. When you download a modification, it is usually a text type file that you read and just follow the steps in editing your existing php or tpl extension files. Sometimes the mods will have additional files for you to add to your forum. Phpbb does a very good job with consistancy and with supplying the instructions step by step. good luck. I will help you if I can. However, I consider myself an intermediate and not an advanced type user.
and no, it does not cost money to get the mods from phpbb site (as far as I know). They're free and usually you can get some free help by asking. However, they do accept donations.