How to launch default web browser from the terminal?

sensible-browser is the command you're looking for.

Searching on Google I found the answer.

xdg-open opens a file or URL in the user's preferred application. If a URL is provided the URL will be opened in the user's preferred web browser. If a file is provided the file will be opened in the preferred application for files of that type. xdg-open supports file, ftp, http and https URLs.

xdg-open is part of xdg-utils package and it's already installed on Ubuntu 10.10.

You can also use:

x-www-browser http://some-url.org

And it will open the URL in the default browser.