Android (4.0+)/WP8: We've seen a number of new personal assistant apps appear recently, but Indigo stands out because it keeps track of your conversations, questions and reminders. That means you can put down one device, grab another, and pick up right where you left off.
Indigo has the features you would expect from a decent personal assistant: The app can get you the weather, add calendar appointments to your phone's calendar or your Google calendar, set reminders and alerts so you don't forget to do something you planned, and help you with basic research. Ask it basic knowledge questions and it can go back and forth with you for ages. The app can also connect to Facebook and Twitter, update those services for you by voice, and send SMS messages for you. Indigo can also look up nearby locations, show you Yelp and Google Places reviews, plot directions for you, and even show you the location on Street View, without you touching the phone.
Indigo is definitely in beta, and has a bit of growing to do, but if you have multiple Android or WP8 devices, it's useful to be able to work on one device, then pick up where you left off with the same reminders and saved data on another phone or tablet. The description at Google Play says the app specifically supports Nexus phones and tablets and the Galaxy S3, but I had no issues with the app on my testing phones, as long as they were Ice Cream Sandwich or higher.