Free IMAP On Gmail
A number of readers are writing in to tell us that Google is rolling out IMAP support for Gmail accounts. Several people say that some of their gmail accounts offer the IMAP option (in Settings, Forwarding and POP/IMAP) and others do not.
Yeah, no limit that I can notice - just downloaded my archive of 1412 messages...
Beside the mentioned Webmail extension http://webmail.mozdev.org/ you could also try FreePOPs http://www.freepops.org/ which basically is an easily extendable Webmail scraper delivering the goodies with POP3.
Disclaimer: I never tried any of those.