Totally, absolutely, 100% depends on the site. The whole point of plugins/widgets is that not everyone needs the same things, and you can use whatever combination suits you. You can browse plugins at the official plugin directory, there are also some default widgets available on every new WP install by going to appearance > widgets in your admin panel.
Whats your site about? If your theme isnt twitter and facebook ready then you should include it in your widgets. There is more for sure but it all depends on your niche!