are there easy to customize/extend forum web apps?

let's say I want to make a forum that has pretty much all the functionality of a typical high end forum, like phpBB2, but I also want to add a few more features here and there. Maybe I want to add user reputation (kind of like what we have here on StackOverflow) and a button to sort threads or posts based on that in some pages.

Needless to say, I also want to do this without reinventing bicycles or rebuilding pyramids from scratch...

What is the professional approach to this? What would you do if you wanted basically a slightly tweaked, advanced web forum?

I would suggest that you take a look at Vanilla Forums:

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I'm biased but I'd recommend looking at Drupal - you seem to want to build a customized system out of existing components and Drupal's module architecture lets you do this quite easily. There are lots of resources on the web for learning how to build community sites with Drupal that a quick Google search will bring up.

You can then use modules like the [User Karma][1] module to create a reputation system


[1]:

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Pligg, open source, seems pretty useful for features such as voting up and down posts

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.

BBpress

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, integrates with Wordpress and allows for plug ins.

Also,

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looks interesting!