AssistantExtensions is a jailbreak app for iPhone 4S that greatly enhances what you can do with Apple's Siri virtual assistant, including voice activation, app launching, translation, and more. Developers can even add their own, custom commands via a new plug-in architecture. That means, unlike the limited built-in functionality Apple provides, many more powerful, and interesting, commands become available.
AssistantExtensions bundles a number of commands together, including the ability to send tweets from Siri, and an interesting "chat mode" where Siri loads additional content so she/he can better have a conversation with you (though the server load must be heavy as it frequently experiences time outs).
For even more functionality, AssistantExtensions lets developers create custom commands and tasks using Siri. You can already use AssistantExtensions to access SBSettings to quickly toggle Wi-Fi, Bluetooth, Airplane mode, etc. Hopefully more and more jailbreak developers start integrating support.
Out of the box, you can use Siri to activate SBSettings toggles or launch apps without even touching your iPhone -- a great way to increase efficiency when using Siri. The only issues we've noticed with AssistantExtensions are random conflicts with SiriToggles, although other users have seen mixed reports.
If you're looking for even more jailbreak utilities, here are the best jailbreak applications for iOS 5.
If you're looking to get started with jailbreak, here are your go-to resources.