I think she is the only famous good looker I am ever likely to bump into in real life. I have seen her twice - once on a railway platform and once pushing a stroller out shopping. Both times I gave her a second “wow she’s hot” look before realising who it was. She had vastly different coloured hair both times.
I didn’t mind Fred like so many other Angel fans apparently did, but her cute glasses went a long way with me. I did think Illyria was a more interesting character, and way sexier.
Kate Holmes is literally the girl-next-door to me. She’s a milf now, and even more beautiful.
jennifer Lien, who played Kes on ST:Voyager, just didn’t do thing for me in the get up they had for her on the show, then I saw a photo of her not made up, and I’ve been smitten ever since. Nice photo of her.
I know I’m not the only one who thinks Tricia Helfer, who plays Number Six on Battlestar Galactica, looks hotter out of character than in character.
Although, truth be told, either one could eat crackers in my bed anytime, know what I’m sayin’?
Straining the OP criteria of actresses about as far as the Pamela Anderson suggestion, I’d say Jessica Simpson.
I recently saw an ad for her doing some magician special, and she was completely done up and hideous. In fairness, it looks like she may have had some unflattering plastic surgery, so we can’t completely blame the stylists or whomever it is that primps her out.
I remember how she looked when she uttered the famous “is this chicken or fish?” line on Newlyweds…hanging out in sweats with very little makeup. At that point, she was possibly the hottest woman on the planet. Painfully hot; we’re talking Icarus territory.
Now? As she looked in the ads for the show I mentioned? It hurts to look at her. But even back then before she mutated, she was far hotter sitting on the couch than when she’d get dolled up for public events.
That’s * chick * to you , ** Operation Ripper ** 
and being a straight chick doesn’t stop me from appreciating the beauty of other women, but in Ms. Hewitt’s case, the cupboard is bare (or should I say the bag of ferret chow?). I will concede that she has a nice rack, though.
I’ll go with Jenna Von Oy.
Summer Glau.
She had some pretty unflattering moments in Firefly. But man, she’s a pretty girl (look at the headshots on imdb versus the Firefly pics…) Of course, “River” was never a pretty character, until she moved. Then, mesmerizing. 
I agree with pretty much this whole thread, even if I don’t know who everyone mentioned is. Women look so much better when they’re real.
Word. Charlize Theron’s uglification in Monster was nothing compared to Sedaris turning into Blank. And it doesn’t hurt that she is one of the funniest people alive.
You know years ago Sedaris was nominated for some kind of “attractive up-and-comer” type of award and when her make up artist before the award ceremony asked her how she wanted to be made up, her response was, “Can you make me look like I’ve been beaten?”
Emily Perkins is known for playing weird, gothic characters in weird, gothic movies. She always looks good in character, but I think she looks even more spectacular in normal mode.
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One of the VERY few female “stars” I’ve seen off camera is Molly Price, who played tough NYPD cop Faith Yokas on “Third Watch.”
She is WAY prettier in real life than she made herself up to be on the show.
Speaking of Tricia Helfer, I watched some the of BSG extras on the DVD and had her out of charater. She was very good looking, but in a very different way than #6. Whereas #6 is “Maxim photop shoot” Tricia was more “very good looking woman you might see in real life”.
I should mention that the May Glamour had “plane jane” chars made up. It was Pam from The Office (US), Dr Bailey from “Grey’s Anatomy” and Mary Lynn Rajskub. I think they went a little overboard with MLR, but she definitely looked different.
Brian
I would find Rajskub adorable as plain Jane, glamour-puss, or nothing at all.
My own personal sighting along these lines: Tracey Ullman. She plays 100 different characters, but she never plays “pretty.” Years ago I spotted her in a restaurant in NYC, dressed entirely in beige (IIRC). Her look was much softer and subtler than she ever appears on TV or in a publicity pic, so much so that I at first had trouble recognizing her. I just thought, “Who’s the pretty woman?”