A few ones I use often:

* Get Markdown link of the current page as [Title](url):

  javascript:void function(){function e(){var e="";if(window.getSelection)e=window.getSelection().toString();else if(document.getSelection)e=document.getSelection().toString();else{if(!document.selection)return null;e=document.selection.createRange().text}return e}var t=e(),n=window.document.title,o=window.location.href,i=[];if(t){i.push("["+t+"]")}else{i.push("["+n+"]"),i.push("("+o+")")}window.prompt("created link",i.join(""))}();

* See current page on the Wayback Machine:

  javascript:void(window.open('https://web.archive.org/web/*/'+location.href.replace(/\/$/, '')));

* Twitter to Nitter:

  javascript:h=['nitter.it','nitter.snopyta.org','nitter.net']; location.href=location.href.replace(window.location.host,h[Math.floor(Math.random()*h.length)]);