GAEUnit is a unit test framework that helps to automate testing of your Google App Engine application. With a single configuration change (it can be completed within 30 seconds), your unit tests can be run in the real GAE app server environment using a web browser.

gmemsess is a secure lightweight memcache-backed session class for Google appengine. It is only suitable for short-term sessions, for providing your own authentication system or shopping cart, for instance. gmemsess is in use in at least one commercial application, and depending on memcache for data storage (rather than the datastore) has now been well proven.

Gawsh is a framework and set of utilites to support development for the Google App Engine.

The first utility is a bulk-download facility to support downloading data from a Google App to a file on the client, in either CSV or XML format. It consists of a client module, run on the desktop, and a server module to be customized and incorporated into a Google App, with a similar structure to the bulk upload components supplied by Google.

Federated authentication is a decentralized security mechanism not tied to any one particular provider, and OpenID is an open standard which implements federated authentication. With OpenID, users can create an identity (username and password [or alternatives such as biometrics, keyfob, etc.]) at any OpenID provider and be able to use that same identity at any online app which accepts OpenID logins. Google has supported the OpenID 2.0 protocol since 2008 in a variety of products such as Blogger and Google Apps

Google App Engine's best feature is being free and easy to get started. From personal experience, if used properly, the free quota is generous enough to serve far more than Google's estimated 5 million page views/month. Here are some tips I've accumulated from personal experience and reading about those of others to help you minimize your quota usage.