HTML5 in GMail
The most interest thing about this is that, considering it works for me in Chromium on Ubuntu (Chromium is the name of the open source project behind Chrome, and the name used for daily-builds, etc) - which lacks Gears - means this must be done using HTML5.
Back at the Wave launch at I/O, Google mentioned how such drag-and-drop functionality was possible with both Gears and HTML5 (Wave is all HTML5). Chrome/Chromium for Linux lacks Gears as the devs expected everyone to be switching to HTML5's local storage, yet until recently the only Google product to use HTML5 in place of Gears was Wave, but a few days back Google said they're transitioning Docs to use HTML5 for offline mode. GMail (on the desktop) however still uses Gears, with no announcement I've seen about HTML5 support - but this Drap-and-Drop being done via HTML5 is hopefully a sign they'll add HTML5 for offline mode to GMail soon (maybe they'll support both HTML5 and Gears so they can move IE6-using enterprise customers from Exchange to GMail).