As a result, there are now hundreds of these things, including a whole bunch of astro related ones. As well as APOD, you can get several different planetarium style viewers, including a free version of Starry Night, widget versions of the Vizier catalogues and ADS literature search, two different solar image viewers, and the latest Astronomical Telegrams. Those links are to the web-pages. To find the widgets, go to the Apple download page, browse, and enjoy. Its rather addictive, as they are so easy to download and try out. As well as all that astro stuff, there are lots of amusing things, like a Universal Translator, a Roman Numeral Clock, and regular stuff like weather forecasts etc.
A lot of this stuff you can of course also get with VO tools like VODesktop, Aladin, or PaperScope. The advantage of the widgets is they take about 3 minutes to understand. The advantage of the VO tools is that you can carry on and do some real work with what you find.
If you want to know how to develop widgets, go here.
In fact, anyone can have widgets by using iGoogle ( http://www.google.co.uk/ig?hl=en ). I’ve also got APOD as a widget as well as ones showing the time in several cities, weather, quick links to wikipedia and xe.com as well as the all-time essential daily Calvin and Hobbes. I even get a theme to the page painted by Rolf Harris 🙂
Google is pretty much the standard now for gadgets. I’ve not tried writing one myself but have heard they’re pretty easy ( http://code.google.com/apis/gadgets/index.html ): sounds much like the Apple ones. In fact, seems you can convert Google gadgets to Apple widgets ( http://www.widgetop.com/downloads.html#AGoogleGadgetTemplate ). AFAIK the Google gadgets conform to the OpenSocial standard as well ( http://code.google.com/apis/opensocial/ ) so any OpenSocial interface (which I’m hoping myExperiment will eventually incorporate) can have Google gadgets added into it.