I know Google Apps do have some offline support with Chrome (gmail, calendar, drive), but all those fall short in any real use. Primarily because it's hard to know what content is available offline and what is not.
Major email clients (for example, Thunderbird or Outlook) do not support major gmail features, namely, "Archive", "Mark as Spam", inbox tabs, labels (different from folders).
Syncing Drive to your computer is entirely possible with Google Drive sync, but even after syncing opening any Google Docs files require internet connection. Same files work offline with web version - assuming those were downloaded.
Is there desktop application offering at least some of these features with reliable offline support, either for Windows or for Mac OS X?