On Monday, Microsoft released a developer preview of Windows Phone 8.1. Anyone who signs up as a developer (free of charge) can now download the update.
The biggest change in the update is the additional of Cortana, a voice-activated digital assistant similar to Siri and Google Now. Other key new features include Start screen and Lock screen customization capabilities, an Action Center for viewing notifications and modifying settings, and an enhanced keyboard that enables "shape writing." It also includes several new enterprise features, like an enterprise VPN, secure email support and other security and mobile device management capabilities.