Most useful  Rails Plugins , Ruby libraries and  Ruby  gems ?
I have seen many site which provides 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 whole lot of time searching for useful plugins and which suits our requirement . 

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

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

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

Thanks 

 
Most useful  Rails Plugins , Ruby libraries and  Ruby  gems ?
===

I have seen many site which provides 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 whole lot of time searching for useful plugins and which suits our requirement . 

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

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

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

Thanks 

 