Basecamp is a web-based tool that lets you manage projects (or simply ideas) and quickly create client/project extranets. It lets you and your clients (or just your own internal team) keep your conversations, ideas, schedules, to-do lists, and more in a password-protected central location.
43 Things is a goal-setting social software web application. It currently has 6,000 registered users and hundreds of thousands of unregistered visitors. 43 Things has 4,500 lines of code that were developed in three months by three full-time developers.
Ta- da Lists is a free online service that implements simple, sharable to-do lists. It features a highly responsive user interface that uses XMLHttpRequest to minimize waiting for the server. Ta-da Lists came from one developer using one week of development time producing 579 lines of code.