I'd have to nominate glium, an "elegant and safe OpenGL wrapper".
OpenGL is, in a nutshell, an absolute nightmare to work with. It has a lot of hidden state, error checking is inconsistent, far from foolproof and often-driver specific, and there's usually several ways of solving any given problem and it's never clear which one is the best approach. It's improved significantly in recent revisions but the API will continue to be full of confusing legacy cruft until Khronos decides to start from scratch.
glium is important because it provides a safe and no-surprises wrapper around the OpenGL API that is as idiomatic to Rust as it can be. It does as much error-checking as possible at the application level, before a misused function call can go into the driver and crash it. It provides RAII wrappers around the various OpenGL data types to avoid leaks and use-after-free.
Most importantly to me, with its companion crate glutin, an OpenGL function loader (required for any OpenGL application), it is written in pure Rust, which means no native dependencies like SDL that may or may not be hard to get installed on the target system. This is especially a problem on Windows, my primary platform, because there's no built-in package manager; native dependencies either have to be bundled with the application or installed separately.
I've never used glutin or glium directly, but I've used them through glutin_window and glium_graphics respectively. They've been my preferred window and graphics backends for Piston for quite some time now, because they just work.