Neovide is a simple graphical user interface for Neovim (an aggressively refactored and updated Vim editor). Where possible there are some graphical improvements, but functionally it should act like the terminal UI.
Onivim 2 is a retro-futuristic modal editor - the next iteration of the Onivim project - combining Vim-style modal editing with the aesthetics and language features of modern editors.
VimR is a Neovim GUI for macOS with the goal of building an editor that uses Neovim inside with many of the convenience GUI features similar to those present in modern editors. VimR mainly uses Swift, but also C/Objective-C when/where appropriate.
FVim is a cross-platform Neovim front-end UI, built with F# (programming language) and Avalonia. FVim features HiDPI support, high-performance, GPU accelerated rendering, remoting and even window detaching.
NVUI is a modern front-end for Neovim complete with animated windows and cursor, smooth scrolling, configurable positions, sizes and colors and more. NVUI is cross-platform, using Qt and DirectWrite on Windows.