I find that since I generally always carry a MT on my person, keychain tools for my needs tend to be super rudimentary. Usually just a beater of a prying tool and a bottle opener (I drink a good deal of bottle beer). A Gerber Shard has been a constant companion for about 10 years. I think it's around 5 bucks.
For the most minimal of keychain cutters, I have really enjoyed my Screwpop. Pretty nifty design that uses a common utility blade held closed with a strong magnet. Plus the front of the tool is a decent cap lifter. I usually carry mine with a serrated utility blade for messy box duty at work. I like how the serrated blade just tears through things even when it gets dull.
The nice thing about the Screwpop is that if you forget you have it on you when going into a restricted area, just dump the 30 cent blade in the trash and you are left with a totally compliant cap lifter.
I usually carry mine off my keys, but I like it enough to spend another 10 bucks to get a second one.