There are several scripts which you can use on your Special:MyPage/global.js or on your project's user common.js (go to your project and type Special:MyPage/common.js), that might improve your editing experience. Here is a short list.

For multiple projects

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Autocomplete

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Start typing a link or template like [[Exa.. or {{Exa and it'll suggest. Use:

mw.loader.load('//he.wikipedia.org/w/load.php?modules=ext.gadget.autocomplete'); //Autocompletes links, templates

Task management

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Add todo list on upper right corner, easy to manage tasks. Use:

mediaWiki.loader.load( '//en.wikipedia.org/w/index.php?title=User:Evad37/ToDoLister.js&action=raw&ctype=text/javascript' );//add a todo list right upper corner of Wiki-page, easy to manage tasks
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Copy section headers easily. Use:

mediaWiki.loader.load( '//en.wikipedia.org/w/index.php?title=User:Bility/copySectionLink.js&action=raw&ctype=text/javascript' ); //Copies section link

Wikipedia

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Prose size count

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Count prose size easily. Yes, it excludes templates, references, categories etc. Use:

mediaWiki.loader.load( '//en.wikipedia.org/w/index.php?title=User:Dr pda/prosesize.js&action=raw&ctype=text/javascript' );//

MoreMenu

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Adds a bunch of useful tools and links ad MoreMenu. Awesome tool. You need to add script in both js and css page. See script page here.

 

WikiPlus

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Edits a page's source code in a popup window. Note: it loads script from outside Wikipedia/Wikimedia. I tested it and it works fine. However please use caution and own understanding. Once installed it'll ask if you want to share some data.

mw.loader.load('https://wikiplus-app.com/Main.js');

Wikidata specific

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Wikidata framework

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if ( mw.config.get("wgDBname") !== 'ruwiki')
mediaWiki.loader.load( '//ru.wikipedia.org/w/index.php?title=MediaWiki:WEF_AllEditors.js&action=raw&ctype=text/javascript' );//Add Wikidata framework