Pligg a fully featured open-source Digg clone. With its simple install, you can be running a digg-style social media comunity in about 15 minutes. It includes its own interpretation of the legendary Digg algorithm, which you can tweak to your hearts content.
Pligg boasts a massive install base, and consequently a large community. A google search for "powered by pligg" returns over 2 million results.
Coded in PHP, Pligg utilizes the Smarty templating engine to provide a high level of customization. Customizeable pligg templates are commercially available, although the complexity of Pligg's templates makes it fairly hard to create one from scratch. If you are working with a client who wants a Pligg site, you are probably best off to make the design elements in photoshop and then pay to have the design converted to a template. It'll probably cost you about $150 for most designs, but will save you a lot of learning if you are not comfortable with Pligg. Social media is an attractive option to offer your clients, so you may want to try your hand at creating a Pligg site just to have it under your belt. You'll save a ton of money using open source software like pligg, and you can pass that savings to your customers.
Pligg also features a rich plugin architecture and many community created plugins. Popular plugins include RSS Import, which adds the contents of RSS feeds to your Pligg site, and AdSense integration.
Pligg requires PHP and MySQL, making it excellent for most shared hosting environments.
Homepage: www.pligg.com
Download: http://forums.pligg.com/current-version/