Open source CMS, nice UI

Stellarium is a free open source planetarium for your computer. It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope.
It is being used in planetarium projectors. Just set your coordinates and go.

Vanilla is an open-source, standards-compliant, multi-lingual, theme-able, pluggable discussion forum for the web.

Infochimps lets you discover, share and sell data of any size, topic, or format.

Atmail is a Linux based Webmail Client + Email solution. Use as an alternative to Gmail or Exchange, and bring your organizations email under control.

The Open Font Library hosts the cream of non-proprietary typeface designs.

Drupal based open source project management.

Fat Free CRM is open source Ruby on Rails-based customer relationship management platform. Out of the box it features group collaboration through task assignments, campaign and lead management, contact lists, and opportunity tracking.

Ringside Networks is focused on bringing the power of social applications directly to your corporate websites and business applications - enabling your global communities to connect and interact like never before.

MAMP is a very easy to install compilation of Apache, PHP and MySQL for OSX

A new open source ecommerce platform that looks really promising. No support from basica Paypal and Google Checkout yet, but I'm sure this is planned.