The Ruby Toolbox: See The Most Popular Ruby Libraries By Usage
The Ruby Toolbox gives Ruby developers a categorized overview of 100 or so different libraries ranked on how commonly used they are. It's not perfect as it only pays attention to projects hosted on Github and the ranking system is based on the number of watchers and forks they have, but it's enough to give you a basic overview of the activity within a certain area.
For example, in the Browser testing section, webrat picks up the #1 spot with 750 watchers and 147 forks, while watir is down at #3 with only 42 watchers and 18 forks. Whether this is a fair assessment or not is down to you. Even if a project has its code on Github, not every project has "culturally" moved there yet, and so the number of followers and forks could be misleading.
As a way of finding out what libraries are at least getting widespread acceptance, The Ruby Toolbox is awesome, particularly in changeable areas like Rails Tagging plugins (acts_as_taggable_on takes #1) and Testing Frameworks (Cucumber takes #1). That said, hopefully you're using better criteria than the number of followers and forks of a project to make key development decisions anyway.