Android/iOS/Web: If you're still on the hunt for a solid Astrid alternative or need a solid cross-platform to-do app, GQueues may be the perfect choice. It's packed with useful features, integrates nicely with Gmail/Google Apps, supports iOS and Android phones and tablets and looks great on the web.
GQueues has all of the features you would expect from a to-do app: recurring items, tagging and categories to keep your lists organised and separate, sub-tasks and dependencies, drag-and-drop reordering, and public task sharing so you can delegate items to other people, whether they're using GQueues or not. GQueues also features "Smart Queues", pre-defined categories that help you see just overdue items and things you have to do today.
While the video above shows you how the app looks in Android, the new iPhone and iPad versions have a similar video tutorial you can check out to see how the system works in iOS. Overall they're much the same: You can use voice input on Android or iOS to input to-dos, and use plain language to schedule events, set reminders, and speak event details. Everything is synchronised with your Google Calendar, and any changes you make sync back to GQueues. The app plays nicely with the GTD productivity method, and there's a custom guide to using GQueues with GTD.
GQueues is free, and connects to a Google Account when you sign up. However, if you want the best features out of it, including the mobile apps, calendar integration, Gmail widget and Chrome extension, you'll need to pay $US25/year for a pro account. You can see the differences between the free and pro versions here. The mobile apps are free, and available in the iTunes App Store and at Google Play for iOS and Android respectively.