Just because you didn't come up with the idea for Twitter, YouTube, NetVibes or PayPerPost doesn't mean you can't run a similar service, right? There is an abundance of Web software clones available, pretty much everywhere (there are even websites dedicated to this exclusively) making whatever big name service you aim to mimic a relatively easy task. Here are a few that I found this morning during my routine web surfing - if you know of a software clones that should be included here (paid software only - no open source, please) then register to comment and we'll add it to the post.
Twitter Clone: ReVou's $399 PHP Twitter Clone lets users engage in micro-blogging. Revou is fully customizable and includes SMS integration, custom API, single domain license, one year of support,d free updates and a 15% lifetime hosting discount from JaguarPC.com
PayPerPost Clone: BlogQuick 1.0 is a PHP PayPerPost Script that allows webmasters to create their own blogging marketplace for $249. Features include text and photos support, built-in promotion system to offer "opportunities", multiple currencies and languages, free updates/support and installation. The makers of this software also have a decent looking AdBrite clone called AdQuick which is worth a look.
YouTube Clone: ClipShare is a YouTube PHP script sells for $199 (the Pro version - the Lite version goes for $99) and offers real-time video conversion (20 video formats supported too, an embeddable flash video player, video and profile rating and more. https://www.clip-share.com
NetVibes Clone: Altrasoft has created an enterprise level AJAX software application for personalized start pages. The software is organized into tabs, with each tab containing user-defined widgets including RSS readers, Notes, Web search, weather forecast, bookmarks, email reader and more.