Mark Lyon has written Mbox & Maildir to Gmail Loader, or GML, which transfers emails in an mbox file into Gmail accounts. Mbox is the format Mozilla Mail and Mozilla Thunderbird uses, so if you use something like Outlook or Outlook Express, you'd have to export your emails into Mozilla Mail or Thunderbird first.

GML screenshot

Works nice enough, though it had some problems with emails containing attachments - those didn't import properly and Gmail couldn't parse the email, so were as good as lost. It is still good stuff because I can now archive my existing email accounts in Gmail (I do have 1 gig of space).

Of course, GML works by forwarding the emails from your mbox file to your Gmail account, so it neccessarily becomes a possible tool for spammers to bomb your Gmail account. Not that there aren't generic email spambots in existence already.

Other similar applications include Pop Goes the Gmail and GTray also seems useful, but I can't get the website to load at the moment.