Code
// ==UserScript==
// @name Stack Exchange: Underlined post links
// @description Underlines all links in questions, answers and comments on all Stack Exchange sites.
// @version 1.1
// @include http://stackoverflow.com/*
// @include http://*.stackoverflow.com/*
// @include http://serverfault.com/*
// @include http://*.serverfault.com/*
// @include http://superuser.com/*
// @include http://*.superuser.com/*
// @include http://stackexchange.com/*
// @include http://*.stackexchange.com/*
// @include http://stackapps.com/*
// @include http://*.stackapps.com/*
// @include http://askubuntu.com/*
// @include http://*.askubuntu.com/*
// @include http://answers.onstartups.com/*
// @include http://*.answers.onstartups.com/*
// ==/UserScript==
var style = document.createElement("style");
style.appendChild(document.createTextNode(".post-text a,.post-text a:hover,.comment-copy a,.comment-copy a:hover,.wmd-preview a,.wmd-preview a:hover{border:none;text-decoration:underline;}"));
document.head.appendChild(style);
About
Underlining post links has been requested and declined. This user script lets you choose.
License
I hereby release this script in the public domain.
Platform
This user script should work in all major browsers. Plug-ins/add-ons may be required.