When we asked you what services you'd like to see liberated, Gmail came up several times. I have good news! Some of those requests are already available. Probably the best way to get your mail out of Gmail is with IMAP. A great place to start learning about IMAP is in the Gmail Help Center athttp://mail.google.com/support/bin/answer.py?hl=en&answer=75725.
There are many IMAP clients available, but I like Thunderbird. It has the added bonus of being open source. Once you connect it to your Gmail account there are a lot of things you can do.
Andy asked to be able to
"Export Gmail email to mbox file. Even better if you can choose to just export messages with a given label, or those matching a search."
We don't have a great answer for the search part of your question, but with thunderbird, exporting a label is easy. Just right-click the folder (labels appear as folders in Thunderbird) and choose 'Import/Export' > 'Export folder.' By default Thunderbird exports messages as .mbox files.
Ludovic PIOT asked:
"We need to be able to import any ThunderBird or Apple Mail archived e-mails into our GMail account."
Once you have the data in Thunderbird, you can drag and drop folders straight onto the [Gmail] folder. Thunderbird will create a new label for you and populate it with the messages from the folder you dragged over.
Jorijn Smit asked for"The ability to merge gmail accounts into one. I have a couple of addresses and it would be great if all these boxes could be merged. Not as in one accounts that checks mail of the other but full intergration of all apps."
For your Gmail messages you can connect both accounts to Thunderbird and drag and drop the messages from one account to the other. Remember to bring your filters to your new account too -- you can learn more about that at http://gmailblog.blogspot.com/2009/03/new-in-labs-filter-importexport.html.
There are lots of other ways to get at your Gmail data with an IMAP client like Thunderbird. Feel free to post your experiences and further questions as comments.
Daniel - First Colonel in the Data Liberation Front