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DataNucleus is used by several companies in their own software. Below are a short subset of those publicisable.

Google App Engine (TM) lets you run your web applications on Google's infrastructure. App Engine applications are easy to build, easy to maintain, and easy to scale as your traffic and data storage needs grow. With App Engine, there are no servers to maintain: You just upload your application, and it's ready to serve your users. AppEngine provides persistence using a DataNucleus plugin for their BigTable datastore.
Tivoli Security Policy Manager uses DataNucleus to work with the underlying database.
Roma allows you to develop enterprise level Java applications with low effort following a real Domain Driven Design approach. It's a new way to conceive the application: anything is a POJO, from the GUI forms to the persistent objects. Roma provides persistence using DataNucleus Access Platform
JFire is the new, powerful and free ERP, CRM, eBusiness and SCM /SRM solution for business enterprises. JFire is entirely free/open-source software, uses the latest technologies (J2EE 1.4, JDO 2.0, Eclipse RCP 3.3) and is designed to be highly customizable. JFire provides persistence using DataNucleus Access Platform
MetaMicro make use of DataNucleus persistence in their products
An online personal finance manager, using DataNucleus Access Platform for persistence
Travel Planning, Vacations and Travel Guides. Uses DataNucleus AccessPlatform in their website
Javelin : a high productivity yet lightweight development tool that allows software developers, analysts, consultants, mentors, educators to drive any Java development through a set of "live" class diagrams. It provides 'zero time' synchronized generation and management of code and the meta-data required for persisting your object model using DataNucleus.
Ridegtop Group use DataNucleus in their products