Mongo is a proprietary object database developed by 10Gen as part of their 10Gen application stack. 10Gen is a startup that is trying to position its application stack as cloud platform for Web 2.0 style applications.
Like many of the emerging Web 2.0 database platforms (SimpleDB, BigTable, SSDS) Mongo looks to improve scalability by simplifying functionality over a traditional RDBMS database. But unlike SimpleDB and BigTable, Mongo aims to do this as a true object database rather than the key/value pair system of the previously mentioned databases.
Mongo is yet unproven and has many hurdles to jump before it becomes an accepted database platform. The first is convincing the development community that they have solved all of the major object databases issues which have prevented them become popular over the last 20 or so years. Additionally a significant challenge will be getting Web 2.0 startups to build their services on top of a platform which itself is a start up (building your stack of cards on another stack of cards) rather than a mainstream database vendor. The available technical documentation is very light on detail and will need to be significantly improved before adoption increases.
The alpha version of their Cloud platformbecame available last week.