To document wiki usage in U.S. K-12 settings, this study examined a representative sample drawn from a population of nearly 180,000 wikis. The authors measured the opportunities wikis provide for students to develop 21st-century skills such as expert thinking, complex communication, and new media literacy. The authors found four types of wiki usage: (a) trial wikis and teacher resource-sharing sites (40%), (b) teacher content-delivery sites (34%), (c) individual student assignments and portfolios (25%), and (d) collaborative student presentations and workspaces (1%). Wikis created in schools serving low-income students have fewer opportunities for 21st-century skill development and shorter lifetimes than wikis from schools serving affluent students. This study illustrates the exciting potential that Web 2.0 data warehouses offer for educational research. (Contains 3 tables, 2 figures, and 4 notes.)