Sharon Stone as Penny’s mom?
The Beav! 
Yes, from Season 5 Episode 3, “The Pulled Groin Extrapolation”
From The Big Bang Theory wiki:
We saw Leonard and Penny learn of the ring in a previous episode earlier this season, as well as the fact that they consciously did not tell Amy. (I forget if it’s because Sheldon asked them not to, or they decided not to tell her on their own, but either way, it was shown to us.)
Oh, I know it was shown to us; my point was it was glaringly obvious to Amy, hence her “You knew about this?” comment.
“Fat” is open to a wide range of interpretation. Penny’s mom could be anything from “pleasingly plump” to “morbidly obese.” The actress who’d play her need not be a porker.
Chubby blondes have always turned me on, be they college- or middle-aged (and anything else in between). :o
Involving oneself in another couple’s problems is never a good idea. They did absolutely the right thing by keeping mum.
Jason Alexander for Leonard’s dad. He’s short and good at playing neurotics.
Do we know anything about the elder Mr. Hofstadter, beyond the fact that he had three children and is now divorced? We know that Leonard’s mom is a shrink-cum-neurologist, but has his dad’s profession ever been mentioned?
Anthropologist:
Easy to remember if you recall Leonard’s middle name is “Leakey”.
Candice Bergen is a bit too old for Penny’s mom. She has also played mostly moms to older children in recent years, e.g., Cuddy’s mom on House. (Lisa Edelstein is 19 years older than Cuoco.) And that’s one of her younger “children”.
IIRC, Beverly looks down on her husband’s profession.
Ex-husband.
How about Cheryl Ladd for Penny’s mom? About the right age and hair.
I bet the big reveal will be either:
1- Bernadette is pregnant
2- Penny is pregnant
3- Amy is pregnant
4- Raj is gay
5- Raj and Emily get engaged
6- Kripke makes dean
Skipped the spoilers, did you? 
Yes I don’t know what the big reveal is and don’t want to know.
I’m thinking of the little old lady who starts throwing up when the main character is talking to her. Kind of looks like Mrs. Berg on Fame but it’s not her. This is if my memory is correct, I haven’t seen that movie since I was a little kid.
I also pictured Happy Gilmore’s grandma as Sheldon’s Meemaw.
How about we figure out what the character should actually be like before we start figuring out who should play her. Is she gonna be some cougar who starts hitting on Raj (causing his mutism to return) as soon as she’s alone with him, high-functioning alcoholic who hates Pasadena because she can’t get booze as good as what she makes in her still back home, frustrated actress trying to relive her glory days when she played Stella in the Omaha Community Theater production of A Streetcar Named Desire, uber-competent farm wife who can butcher a hog in eight minutes, or something else.
Then we can start throwing out names for actresses. Details matter.
TL;DR: Bonnie Hunt
Since Cheryl Ladd is from South Dakota, which is right above Nebraska, and has been an actress for quite some time, she could be most of those things.
They’d have to pad her.