REX is what the developer calls a "streamlined take on chess": simpler, easier to learn and quicker to play (5-10 minutes), but still with some scope for strategic fun.
The board is a compact 5x5 squares. Each player starts with only 5 pieces of 3 different types, each of which has its own movement rules.
These rules are simple - essentially, they're able to move up to 1, 2 or 3 squares - and a built-in tutorial explains every point. Even if you forget, there are hotkeys to remind you which piece is which, and where it can move.
There's support for playing against the computer, with four levels of AI, or a friend.
REX has been developed using the Unity game engine, which means it's very well presented. An opening screen asks you to select screen resolution, graphics quality, target monitor and optionally set your hotkeys. The graphics are great, with the board and pieces displayed in a gorgeously rendered 3D view (even the wood of the surrounding table looks ultra-realistic).
Please note: we found REX sometimes paused on launch, perhaps for several seconds. At first we assumed it had locked up, so closed the program down and tried again, but eventually we realised there was just an odd delay. If the same thing happens to you, wait, it'll probably fire up after a few more seconds, and there are no more pauses once it's up and running.
REX isn't chess, but it's small, simple, easy to learn, quick and fun to play and entirely free, and that's good enough for us.