ยป WonderCMS
Free and open source CMS without a database. One of the smallest (5 files, 20KB) flat file CMS.
Built with PHP. Started in 2008, meant to be a really tiny and simple alternative for creating websites. It doesn't require any setup (1 step install).
Yahoo! WiZaRdS league
At the ages of 11-16, created biggest pool gaming league on Yahoo, with over 10k members.
mufi.org
Website for sending out Google+ invitations to early adopters. 15k+ invites sent in 2 weeks. Amazing community response with support and donations.