Archive notice: This article was originally published on August 6, 2019. Links and embedded videos are preserved as part of the historical record.
At SIGGRAPH 2019, Autodesk presented Arnold 5.4, the current version of the renderer. The update comes with new Bifrost support, Arnold GPU expansion, a new Clip Geo shader and more.
Arnold 5.4 feature highlights
In the last 5.3 update, Autodesk released Arnold GPU (Beta) for the first time, with support for Nvidia RTX, various shading networks, hair, SSS, atmospherics, instancing and procedurals. The new version adds support for multiple features, including Open Shading Language, OpenVDB, volume displacement and Volume sample shaders, Randomwalk v2 Subsurface Scattering and more.

You can find a detailed, up-to-date list of all supported functions of the GPU beta here.
Bifrost support for Maya
With the new multiphysics simulation tool Bifrost in Maya 2019.2, effects such as fire, smoke and explosions can now be created. The update enables a pixel-accurate preview of the effects in the Arnold viewport. The renderer supports all Bifrost geometry types: Volumes, Strands, Instances. We compiled an overview of Maya 2019.2 and Biforst here.

Clip Geo shader and more new features

A new Clip Geo shader is intended to simplify cutting out objects. The shader cuts out the entire geometry based on assigned shapes. Objects can be selected and separate shaders used for the intersections.
Other new features include a faster convergence rate with adaptive sampling, less noise during progressive sampling on CPU and GPU, and improvements to Alembic and MaterialX support. You can find all the new features in 5.4 in the release notes here.

Availability
Arnold is available as a monthly standalone subscription (70 euros) or in the Autodesk Media & Entertainment Collection. The subscription includes free plug-ins for Autodesk Maya, Autodesk 3ds Max, Houdini, Cinema 4D and Katana. Arnold GPU is currently available only for Windows and Linux. More about prices and licenses here.
Further information: To the Arnold website