Applets are small programs that you attach and start from the GNOME's toolbar. See picture: http://www.futuredesktop.com/audio-recorder/recorder-applet.ogv
My favourite applets are:
rec-applet: Smart audio recorder applet. You can get this from Launchpad.
clipart-finder: Find and download clipart to your computer. You can get this from the Launchpad.
1) Can I use these applets in UNITY as is?
2) What changes I have to make to applets' code (rec-applet and clipart-finder) to make them usable in Unity?
3) Where is the applet-specification for Unity?
I think you can't. Unity only shows notification area and indicators. So if you port their code to use indicator-applet, they will work.
Since both shells encourage you to position launchers closer to the workspace (using the sidebar in Unity and the favourite applications well in Gnome Shell), many of the more common panel applets are close to being obsolete (since they usually just launch things with their main windows appearing close to the launchers). Some cleverness will finish the job.
Both shells are going for better defined, stricter methods for extensions like panel applets. Ubuntu's indicator stuff fits that territory; indicators serve a single use case and they go in one place. Unity also has a feature called Lenses. By default, you have two Lenses in Unity: Applications and Files. In the future, you will be able to install lots more to do all sorts of cool things (like browse Youtube). There is some info about that on the Ubuntu wiki.
As the new shells evolve, additional extension points might emerge.
I'm not sure about rec-applet, but clipart-finder looks like it could be integrated into Unity via the Lenses API.
In the Lenses API session at the recent UDS-N Neil Patel demoed a youtube place he hacked up over the course of two hours. It was sweet, and looked like it would be an excellent match for clipart-finder.
Ok, neither Unity or GNOME Shell will have applets as we know them from GNOME 2.x. Applets are simply small GTK-applications, so if I can open the application window nearby the launcher button, then it would be a great solution.
I hope that Unity can help us with:
1) Find out the location of the launcher button. My
applet applicationGTK-application will then create a window close to the button's location. The window is often small, so opening it in maximized mode or in the center of the screen would be very "rude".2) To indicate status or advancement, the GTK-application may want to modify its launcher button. Can we change the launcher's image or color, or even draw on it?
3) Flip-flopping the window. Clicking the launcher button will show/hide the application window. This will mimic the original applet-launcher.
I want to make rec-applet and clipart-finder rock, also in Unity and GNOME Shell.
If there is a Gnome Panel applet that you absolutely must use and it isn't feasible to get an indicator to replace it, then this might be a workaround:
GNOME applet only runs on GNOME panel. As unity will not use GNOME panel, so it's not possible to run applets unless canonical figure out some way to work around
There are some indicators being developed by the gnome2's applets developers. Follows a big list of them:
What Application Indicators are available?
Another alternative is to install Cairo-Dock on your Ubuntu Unity. You will find it in the Ubuntu Software Centre as GLX-Dock. This is a very cool application with a lot of it's own applets, like weather, disk usage, system monitor, etc.
still to this date I had this problem,
but finally we can run gnome applets in unity as explained on this question.
basically you run
gnome-panel, remove what you dont want from it with alt+super+rightClick and add what you want too; now I can finally access the "cpu frequency scaling monitor" after more than a year...