Reviewed in Form+Function
I never really needed a personal assistant app, mainly because I don't really do anything. Regardless of that, Indigo is lightweight and user-friendly, and does a pretty decent job of handling various requests. For those who just want a laugh, it can do jokes and riddles too, though it's no Samantha.
Pockets6794: Super cool PA app that is more Siri than Siri and syncs to all your devices.