Jeff Waugh

Login Style lets you restyle your login page with a custom CSS file.
Note: It hasn’t been tested on versions earlier than 2.1, but may very well work.

Login Style

Emanuela Castorina

This plugin in a multisite installation shows the ID of the current site/blog in the admin bar menu. The ID of the current site/blog is linked to the site info page. 
It adds also a custom column with site id information inside the sites list table. 
It can be installed only on a multisite.

Wp Show Site ID

This plugin in a multisite installation shows the ID of the current site/blog.

Login Style vs Wp Show Site ID