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For now, the list of collaborative gadgets includes: to-do lists, notebooks, crossword puzzles, birthday reminders, but we'll see more interesting ideas as gadget developers can easily add this new feature.
"It's now possible for multiple instances of a gadget - on the home pages of different users - to access the same user-preference data, the part of the gadget state that is hosted by iGoogle. (...) It's a simple sharing model - last write wins, and a reload is required to see changes made by others."
To share a gadget, click on the small arrow from the title bar and select "Share this gadget". For most gadgets, you can invite your contacts to use them and optionally send your settings, but collaborative gadgets have a different dialog:
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