free dlls, libraries, or plugins to make development easier

Question

language :: C#

windows forms desktop application

 

We are working on some desktop applications (library management, shopping centre, retail management). How can I use the dlls, libraries or plugins to make my development process easier and faster. Please suggest me the lists of them.

 

eg.licensed products telerik, dhtmlx etc.

Note:: the dlls, libraries or plugins should be free to use and download