Gadgets is a wiki extension that has to be enabled by request on wikis here on Wikia, but in my opinion, it should be a default option for all wikis, because the sheer amount of things it can do can be invaluable for all wiki founders and administrators.
One of the most useful Gadgets ever created (and can be found on Wikipedia) is the References Tooltips gadget, which allows the user to see a hovering tooltip that displays a reference/citation in a pop up window when a user rolls their mouse over the reference, instead of forcing the user to scroll to go to the bottom of the page to see what the reference/citation is saying in more detail.
Wikia is not the same as regular MediaWiki, so some Gadgets may have to be modified or Wikia specific gadgets may have to be made for specific features, but using gadgets, the user experience can be greatly enhanced with many great features, like adding links to commonly used features, adding drop down menus for commonly used templates to be auto inserted into articles, and even administrator tools like quick page deletion and bot like tool that allow administrators to perform all types of maintenance much faster than by manual editing.
Gadgets from other wikis can be easily imported if you find one on another wiki that you wish to use on your own, and many useful Gadget scripts can be found on the Wikia Developers Wiki.
In addition, gadgets can also provide user specific skin modifications to get around features of Oasis you may find limiting, only unlike user specific CSS/JS style sheets, these modifications can be easily picked and chosen by any member of your wiki.
In conclusion, the Gadgets extension can be highly powerful and extremely useful, and while it does require some coding knowledge or at least requires some outside help to set up, it can be an incredibly useful addition to your wiki and can provide all users with a wide array of wiki enhancements they can pick and choose from.