iOS (Jailbroken): Since iOS 5 was jailbroken, we've seen a steady stream of Siri hacks. But if you're looking for an all-in-one utility, AssistantExtensions is the best place to start.
AssistantExtensions comes packed with Siri commands for SBSettings, a Twitter function, YouTube search and more. The nice thing about AssistantExtensions is that it can be expanded with additional tweaks from developers and it won't conflict with other Siri hacks. Extensions can be downloaded directly from Cydia and integrated into the AssistantExtensions, so you won't have to choose between two Siri hacks. It's a free download, and you can find it by searching for AssistantExtensions in the BigBoss repository.