Vote up/down on comments so most voted comment get shown at the top

I don't know of one off the top of my head, but entering "type:project_project vote" in the search box above should help you locate possible modules.

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"Give a man a fish and you feed him for a day.
Teach a man to fish and you feed him for a lifetime." -- Lao Tzu
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I'm actually just starting to work with a module called Comment Bury/Promote found at http://drupal.org/project/comment_bury_promote

This seems to be what you are looking for, however it is still being developed and is only available for Drupal 5x. I'm in the process of trying to figure the actual promote feature. Maybe you can shed some light on the issue as well.