Grate is the open-source, reverse-engineered Tegra driver that's been happening for a while. They've made progress with an open-source DRM/KMS driver and also dabbled in a Mesa 3D driver and X.Org DDX driver. The future is ultimately bleak though for Grate given that with Tegra K1 and future Tegra SoCs they have reached the point of being derived from the mainline NVIDIA GeForce graphics architecture.
Early Tegra SoC GPUs deviated a fair amount from the "upstream" desktop graphics architecture but with the forthcoming K1 they are closely aligned on a Kepler-based solution. NVIDIA's mainline binary driver is able to support the Tegra K1 rather than having a separate driver and this will also be the case for Nouveau.
A few days ago NVIDIA pushed initial Tegra K1 support to Nouveau in the form of DRM driver patches. With this closely-aligned architecture, the more mature Nouveau driver will be able to handle this upcoming Tegra hardware in place of Grate.
Anyhow, for those curious about the state of the open-source Grate driver, there's PDF slides from the presentation last week at FOSDEM 2014 by Erik Faye-Lund. While it's not new news, one of the Grate developers, Thierry Reding, is now employed by NVIDIA Corp and continuing to work on the company's open-source projects.