drupal.org
I was never quite happy with the old phpnuke site. Much of the site took a lot of php hacking to get it to look and function right. There were also ongoing concerns about security. I've been hunting around for a while now for the right replacement and fortunately came across a blog that mentioned drupal was worth a look. And here it is.
It still takes a fair bit of effort to install the right modules and modify the structure BUT I have only had to change a single character of php code to customise it. It has a much more professional feel and the project module has a great bug/feature tracker. The selection of modules is very impressive from image galleries to wiki collaborative editing to RSS and news aggregation.
The transfer from nuke wasn't too painful either with this phpnuke to drupal migration script which copies users and forums to the new database. There were a few errors and mangled post threads but overall it was fine. It's worth taking a look...

wordpress
Check wordpress.org... Nice and slik weblog software with loads of add-ons and plugins available...