They've had experiments offline for a long time... Gmail, Google Calendar and Google Reader used to have offline mode through Google Gears - though a while ago

They've had experiments offline for a long time... Gmail, Google Calendar and Google Reader used to have offline mode through Google Gears - though a while ago they yanked it from all browsers apart from Chrome to foist the latter onto people. Of course there is also POP3 and IMAP for Gmail. Also, Google Maps used to have a prominent offline caching mode, but the hid it away more and more.