Most useful Rails plugins, Ruby libraries and Ruby gems?

Question

I have seen many sites which provide the whole list of Rails plugins, Ruby libraries and Ruby gems, but we hardly use few of them and some may not suit our requirement and we spend a whole lot of time searching for useful Plugins which suits our requirement.

I have created this poll, people can post useful libraries, gems and plugins which they have come across. It would be great help for newbies like me and to the entire Ruby on Rails community.

Note: to keep this poll as useful as possible, please remember:

• Post only one library, gem, or plugin per answer
• Mention the name of the library, gem, or plugin which you find it useful.
• URL of the location of the resource
• We don't want duplicate answers, so before posting check if the library has been mentioned already.

Edit: Any new plugins/Gems for Rails 3 ?

Thanks!

Answer 1

Will Paginate - essential for pagination.