Is there a single place where I can find all standard and custom widgets for dexterity field.
Standard widgets can be found at following link
http://developer.plone.org/reference_manuals/external/plone.app.dexterity/reference/widgets.html
BUT, there is no single reference where I can check all possible custom widgets community has developed before I start thinking to develop widget for our user-case.
The list in the manual is intended to be that single place. Anyone should feel free to contribute to it on github: https://github.com/plone/plone.app.dexterity/blob/master/docs/reference/widgets.rst