Ok..it's a tired old topic I know. And I have weathered many a storm with mine, bitten my tongue and toed a tightrope line more times than I ever thought I could..but now that she's here on another one of her annual/biannual visits, the tension in the house has reached epic levels. I don't think I've ever felt more suffocated!
She has just left for a couple of nights with a relative of hers, and I'm literally feeling euphoric. Have been mentally listing the things she does when she is here..and wondered if she truly is unique, or are there other mils like this out there 
Firstly..not a helpful presence in my house, but nor would I expect. She is always very much in guest mode.
So that's an aside...and I wouldn't have an issue with that, if I enjoyed the company. But I don't.
Here are some examples...
Stalks me 24/7...an intense and extremely tense presence, looming over my shoulder all day long. What are you going to do now?..don't you want to do this / that/ or other? On the hour every hour.
Constantly asks in a haughty poshest voice you've ever heard tone..'do you think that's wise?' My teenage daughter tries to avoid her now, as everything she does has this remark..and she's threatening to go stay with her friend for the rest of her trip.
Has no concept of personal space. Walks into our bedroom at 7am on the weekend, and haughtily asks...shall I pull your blinds for you? 
Leaves her bedroom door wide open every night, and it opens right onto ours. It's all a bit cramped up there! Walks in and out of our room, and my children's, as and when she pleases.
Wants to help me make my bed?!? My 7 year old makes her own bed!
Is paranoid about everything the kids do...should you be doing this/that or other? Don't you think you should be doing this/that or other?
Constantly makes indirect references to the fact that DH works long hours and I'm mostly sah. Scrutinises what I spend, what I cook, what I do with my spare time. Nothing is sacred.
Is a self obsessed hypochondriac who I'm 100% sure feigns things to get attention from her son sometimes.
Still 'takes her girls' to the doctors with every little thing... but they are both in their 40s 
Takes authority over my children, tells them what they can and can't do..even while I am stood there!
Upset my 7 year old when she told her categorically, she would not be getting another pet..as it's bad for her allergies. Only we as a family, have already agreed we will be replacing our last pet, and had just been to see one we were interested in! But oh no...mil says no, we can't have one! 
Thinks alcohol is poison, and looks down her nose at us, whenever we dare have a glass of wine.
A friend popped over to say a quick hello, and she had a tiny spritzer which I filled with ice, as she was driving. Mil took the liberty of leaning over and pointing at her glass, and haughtily asked her if she thought that was a good idea, given that she had a car outside. Friend felt uncomfortable and legged it pretty quick!
She has often upset various friends & family over the years, everybody thinks she is stifling and uncomfortable to be around. When she is here, I have actually questioned whether DH is worth it, it is so unbearable, and he can swan off to his office for 15 hours and leave me to it.
Anyway...I'm sure there are many other similar breeds out there...but it's been good to offload! 