Highly-rated Astrid Tasks offers one of the most thorough to-do lists around with the ability to share lists and synch with Google's ( GOOG) own (and incomplete, I must say) Tasks. With the $1.49 Astrid Locale add-on, you'll get alerts on tasks as you near the vicinity. The one problem? It's not available for Apple ( AAPL) users, yet (they're reportedly working on it). iOS users should take check out Remember the Milk, which just added compatibility with Siri, Apple's new voice-activated concierge. Astrid Tasks available on Android (free): Check out the Astrid Tasks app here. Remember the Milk with Siri (free): Check out the Remember the Milk app here.