It's sooo slow. Honestly, it'd be x100 better if it had fewer features, but was wiki-like to make up for it. With hyperlinkable drawing elements, and public maintenance via RecentChanges. Text is a pain on here.
An Ajax enabled online calendar. Drag and drop events to change dates, drag the start/end of an event to create multi-day events, create and edit events without refreshing the page, all with an iCal style interface.