Hot as in cool: Etherpad. Almost one of a kind, taken over and closed down then open sourced after a large backlash. I think a good additional web concept to join wikis and forums and sidebar-chats.<p>Are you looking for something to use or something to contribute to or what?<p>If you really care about popular and trendy, get some real data e.g. from <a href="http://sourceforge.net/softwaremap/" rel="nofollow">http://sourceforge.net/softwaremap/</a> and be surprised that the most popular of the past week are a package of Microsoft Fonts and an ERP, Accounting and CRM app. The most popular of all time are eMule and Azaureus file sharing and Sourceforge.net itself and the Crystal Space 3D SDK.<p>In fact the most webby looking one in the lists seems to be PHPMyAdmin, of all things.