Ad Logger allows you to log clicks on iframe adverts and other social network buttons.
ImageFX allows you to add filters to your WordPress images uploaded through the normal media uploader.
WP-Tweetbox gives you a widget to show a fully customizable Twitter-searchbox in your sidebar.
wpsc Support Tickets is an open source help desk and support ticket system that allows you add support ticket capabilities to any WordPress site.
UI Labs offers experimental WordPress admin UI features with the aim of building upon and enhancing the default WordPress User Interface. These are unofficial core UI experiments – who knows what could happen?