"Pop-Goes-the-GMail" (known at the time as PGtGM) was a program designed by Jon Barker that allowed GMail to be used in conventional e-mail software. The program interfaced with GMail's servers and translated them into a POP3 protocol that programs like "Outlook Express" could easily use. PGtGM also hosted its own SMTP server so you could send e-mails without opening GMail's web interface. I helped with a bit of minor testing and I also helped him moderate the support forums.
PGtGM released as open source, and the original developer, Jon Barker, quit working on the project. A new developer appeared and started developing "GPopper." (http://www.imizzy.com/)
Google now supports POP3 natively, so the use of PGtGM/GPopper is defunct.