Question: How can I add an icon to my Rails application (to show in tabs when opened in a browser, etc.)?

Answer: 
Just so people know, the "document root" where you place your favicon.ico is the "public" folder. As a Rails newb I thought it would be in the root of the entire application. 

Also, you may have to clear your cache for it show up.
After reading and trying all this answers without success I ended up doing this

1. Add a file **favicon.png** to your **public/assets** folder
2. In your **$/app/views/layouts/application.html.erb** modify the content of the `<head>` tag adding this `<link rel="icon" type="image/png" href="/assets/favicon.png">`

No need to change the web\_server configuration file (nginx, apache, etc) no need to precompile the assets. 

Just stop and run/debug your web site, clear the cache from your browser and reopen the page. It should work