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

Answer: 
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
Put the `favicon.ico` under your `public/` folder and then add `<%= favicon_link_tag '/favicon.ico' %>` to your `<head></head>`

Then if you try it and doesn't work out, even after you cleaning browser's cache, you should try run the server at a different port. By default, rails runs the server at port 3000. 

Try changing the port to something you haven’t used before – run the app as:

RAILS 3: rails server -p 12345

RAILS 1/2: ruby script/server -p 12345