wpyb

To prevent that usernames are publically visible, the username in the author’s permalink is replaced with the author’s ID. 
In case the page is invoked with the default permalink structure, it is displayed “Page not found”.
Additionally it is possible to modify the author base. It is recommended to make use of this plugin to change it, but it also works with other plugins that have the functionality to customize the author base.

Change Author Link Structure

To prevent that usernames are publically visible, the username in the author's permalink is replaced with the author's ID.

Dekisugi

This simple and lightweight plugin come with a widget that display the word Welcome along with the username of the current user. This is to give subcribers a sense of warm welcome and hospitality, when they come back to the blog.

Welcome User Widget

A plugin with a simple widget to display welcome message along with the user_login of the current user.

Change Author Link Structure vs Welcome User Widget