Many bloggers out there (including me) use the JAW Popular Posts widget plugin, that displays the most popular post somewhere in the sidebar. This widget requires Popularity Contest plugin in order to run. The problem is that Popularity Contest does not run on WordPress 2.5 (it causes a fatal error when activated). The fix is very easy (you just need to edit one file).

If you cannot wait until Alex King releases the next version of the plugin, you just have to download the latest version (1.3b3) of the plugin and

• open the popularity-contest.php file with your favorite editor
• find line 59 and replace

  require('../../wp-blog-header.php');
  

  with

  require('../wp-blog-header.php');
  

• upload the file to wp-content/plugins/popularity-contest (or where the plugin files are located)
• activate the plugin

If you are installing Popularity Contest for the first time on a WordPress 2.5 installation, you have to create some database tables first. Extended instructions on how to manage it can be found here.