Today I've received a question from a friend (I've sent to him yesterday a new Gmail account) and I want to report my response here, because I think that most of you could have the same necessity and don't know how to solve it.
The question was "How can I move all my mailbox on the mail client on my pc into my new Gmail account?"
I had the same problem when I've opened my Gmail account... I want to transfer all my Inbox from my mail client (Thunderbird) to Gmail and to solve this task I've search the net and I've found a great tool written by Mark Lyon that performs exacrtly what I need: Gmail Loader.
Thunderbird uses the standard mBox format to store the email messages (the messages are stored on a unique file on your hard drive). Gmail Loader is able to read this file format and forwarding all your emails to the Gmail account, without deleting your files from the original source. With this tool, it's really simple and quick to move all your Inbox to Gmail... try by yourself! 
Today, the GMail Loader is a graphical, cross-platform, Python based utility that supports two mBox formats (Netscape, Mozilla, Thunderbird, Most Other Clients), MailDir (Qmail, others), MMDF (Mutt), MH (NMH), and Babyl (Emacs RMAIL). The only missing thing is the support for Outlook format (PST) (I understand that this could be a big lack for lots of you, but I'm not an Outlook user so this tool was exactly what I want to have).
I hope that, if someone of you maybe a day have the same problem here reported, now can understand what type of action to take.