How to get back the standard application in Notification Center in Mac OS X?
Clicking the 'Edit' button at the bottom of the 'Today' panel in Notification Center will show the widgets which have been disabled. Clicking the green plus circle icon beside any of them will re-activate it. When all the widgets you want are active, just click 'Done' button on the bottom-left.

As far as I know the following is a list of the standard widgets supplied with OS X Yosemite


Today
Tomorrow
Weather
Reminders
Calculator 
Social
World Clock
Calendar
Stocks