According to Apple’s very own guidelines for developers and to its marketing literature, widgets should be a breeze to install and use. For a vast majority of users, that is true and nothing beats the convenience of clicking on a link on a site to find, mere seconds later, that it added that one long missed bit of functionality (from tracking subway trains to downloading pasta recipes) to your Dashboard.
Unfortunately, in some rare cases, widgets may fail to install themselves properly or to appear in Dashboard as expected. Curiously enough, this «issue» seems to be more due to a lack of information and detail in the help files than anything else — as we are going to see that the workaround is simple.
Installing a widget
Should Safari be set up to automatically open «safe» (notice the quotes) files, it should automatically detect that the file you just downloaded is a widget, move it to the appropriate folder (usually /Library/Widgets) and add it to your Dashboard tray, ready for you to use.
Should that function be disabled or should the widget be improperly packaged, though, the installation process will not happen automatically. In that case, you simply need to manually uncompress the widget (if applicable) and move it to the traditional “/Library/Widgets” folder — much like you sometimes need to create folders in your Library to add plug-ins to an application.
Widgets located in this folder will be available to all users of a computer and should be automatically picked up by the Dashboard tray whenever you open it. You might need to force the widget list to scroll twice (by clicking on the little arrow buttons) to clear any cached images but that should take care of it.
In some instances, a log out / log in cycle will be required in order to force Dashboard to refresh the tray — adventurous users will perform a “killall Dock” in terminal, as the Dock is the parent process of Dashboard but that is a bit like using a chainsaw to open your iPod socks package…
Uninstalling a widget
Uninstalling a widget is as easy as grabbing it through the Finder and deleting it like you would any other file. To disable a widget without deleting it, simply change its extension — “.wdgt.off” is a favorite of mine but you can get as creative as you wish.
The same “refreshing” tricks apply for the uninstallation than for the installation process. All in all, the Dashboard drawer is very good at reflecting the status of your system but might need a bit of help from time to time.
A word of caution
This does not strictly relate to installing and uninstalling widgets but the question seems to be asked often enough. Apple did build security safeguards into Dashboard but widgets are potentially powerful applications. You should exercise the same caution when downloading widgets than when downloading other kind of applications and executables from the internet. Always prefer trusted sources and, if at all possible, use checksums to ensure that the file is good.