Accerciser
Accerciser is an interactive Python accessibility explorer for the GNOME Desktop. It uses AT-SPI to inspect and control widgets, allowing you to check if an application is providing correct information to assistive technologies and automated test frameworks. Accerciser has a simple plugin framework which you can use to create custom views of accessibility information. Full documentation can be found in the Official Accerciser Manual. For a demonstration of Accerciser and PyATSPI (Python-wrapped access and usage of AT-SPI), see this article. For an excellent walkthrough from the author, see the article titled Make Your Application Accessible with Accerciser.
Accerciser has effectively replaced the older at-poke tool.