Described as a "massive update," Unity 5 features a "completely overhauled audio system," physical shaders and enhanced real-time lighting. Additionally, the update delivers an ad-sharing network that should make it simpler for developers using Unity 5 to cross-promote mobile games. Unity Technologies also hopes to expand the engine's already massive reach with the addition of support for the WebGL graphics API.
Unity 5 will be available for pre-order from the Unity Technologies store at some point tomorrow. Helpfully, paying for the engine early also grants users access to Unity 4 and all of that engine's remaining updates.