NoSQL

Simple DB

 * http://www.ryanpark.org/2008/04/top-10-avoid-the-simpledb-hype.html

Mongo DB

 * http://www.mongodb.org/display/DOCS/Geospatial+Indexing

Riak

 * http://seancribbs.com/tech/2010/02/06/why-riak-should-power-your-next-rails-app/