I am incredibly jealous of what Ben Milton has managed to do. I've been searching for my ideal RPG rules for decades, and never found anything that hit the right balance. I've tried every retroclone and microclone you know, and while they all did some things fantastic and most everything else great, they are generally too restrictive and bogged down in minutea for my tastes. I've been searching so desperately for a good "0e" style system, trying and failing to write three of my own, that I was just plain frustrated with the whole industry. But upon finding Maze Rats, I immediately knew it was something special that just does everything so well.
Don't let the short length deceive you. The first time you read through a short paragraph explaining a concept, you might think it's nothing special. But the more I use these rules the more brilliant I discover their simplicity to be. This is not just a stripped-down set of rules; each sentence is very carefully crafted and each rule and even each random word is there for a deliberate, play-tested purpose.
My first reaction to the random lists was to roll my eyes, thinking "oh, I'm creative enough to not use those; what a waste of space". But upon actually using them, I see the brilliance of it. As Mr. Milton says in the text, random, external ideas are a huge gushing fountain of fun. It will all be filtered and refined through your own mind anyway, of course.
My only nit-pic is that, while in general the magic system is brilliant, it seems a little too chaotic to have every spell slot get filled in with some new random spell every time the caster replenishes used slots. I'll probably home-brew something that allows the caster either more options beyond the single spell formula and name per spell slot, or gives them a few more slots (in either case, probably adjusting the general power of spells down to compensate).
But overall, this is easily the best old-school style RPG product I've ever seen. Others are more extensive, and indeed others are quite clever and interesting in many ways. But Maze Rats is head and shoulders above the rest, as it focuses entirely on producting a great game experience. If a physical deluxe version ever came out, perhaps with some optional rules, artwork, and a sample adventure or two, I'd buy it for, say $10 in an instant, even if it were only double the pagecount it is now. :)