I've been working on and off for some time on a roguelike game called Sanguine which uses Gosu for the UI.
It's not complete but you can wander around, hit things, level, get, drop and equip items. Maps can either be handcrafted or procedural generated using a BSP algorithm. The one thing I really like about the design is the idea of an Effect - essentially something which is applied to a game object e.g. a weapon doesn't have a damage etc but rather it has a collection of effects, some of which are applied on strike. Makes extending really easy and items simple to implement.
It might run a bit slow but that will mainly be because the map view code needs optimising (a simple job I just haven't gotten around to yet), it runs fine for me under Ruby 1.9 on a 2 year old Macbook.
Gosu made the input and tile rendering simple. Having bold-only fonts was a slight pain. Big thanks for Julian and all the other Gosu contributors for making such a great library.
Having just started a new job development is currently suspended but hopefully I'll finish it one day. If anyone gets any use out of the code I'll be happy.
One final thing - this is the first application I ever coded in Ruby, hence it's not as Ruby-ish as it probably could be and I've probably done some strange things. On the flip side I've tried to lay it out logically and comment so it should be obvious how it all works.
There source can be found here: http://github.com/salin/sanguine