Mouse Guard is an excellent game, the best work that Luke Crane has done. It is such a good game, it has inspired a number of very interesting hacks.
My favorite is Realm Guard, which is a hack focusing on the Dunedan in the north of Middle-Earth early in the Fourth Age.
The second, amazingly, also ties directly into one of my interests. It is called Lions of Judah, and has come out of the UK Burning Wheel community.
I’ve read through Realm Guard and can say that except for a couple typos I noticed on the first read-through, it is excellent work. I was impressed and interested, and might actually be running it this Sunday afternoon. We’ll see.