What are the differences between the various OpenID widgets available?
The first and the last link you provided takes to the same project page.


The OpenID Selector module replaces the OpenID login form provided by openid.module (an optional Drupal core module) with the same OpenID selector used, for example, on Stack Exchange sites. Though additional modules, it integrates with login provided by:


Facebook
Twitter
LinkedIn
vkontakte.ru
mail.ru
Windows Live ID

The Loginza module replaces the Drupal core module for OpenID, as it implement OpenID authentication, and it allows to authenticate users using the login provided by the following sites/services:


Google Account
Yandex
Mail.ru
Vkontakte
Facebook
Twitter
Loginza
MyOpenID
WebMoney
Rambler
Flickr
Last.fm
Verisign
AOL
Steam



The other difference between the modules is that the Lorenza module is for Drupal 7, while the OpenID Selector module has versions for Drupal 6 and 7.