Well, since it's strayed a little from the intention of the OP which I believed was "name actresses that appear in a lot of sex scenes" to "name what film you can see what actress' tits in" I might as well weigh in.
Sharon Stone has been mentioned and the grab of her infamous flash from Basic Instinct but the shagging in that between her and ol' Douglas is really good, and I have a real soft spot (or hard spot) for the opening "mysterious blonde woman" scene too, which also featured one of my earliest wankpieces, although I didn't find out until years later who it actually was in that well-shot scene.
Back to Shazza, I highly recommend her showing her whaps and/or fucking with Russell Crowe in The Quick And The Dead, Billy Baldwin in Sliver, Sly in The Specialist, and a clothed (but still intense) riding of Richard Gere in Intersection. Her "good bits" in Blood And Sand and Casino are also great, and I believe there are some pretty filthy scenes in Basic Instinct 2 but somehow I never got round to watching it, even just the fucking.
If a similar vein to Sharon & Richard, "sex" is more important than "nude" I actually quite like the brief shot of Cameron Diaz humping Keanu in Feeling Minnesota, for nothing more than her asking "did you come for me?" LOL. There have been a few mentions of Jennifer Connelly but I don't think anybody has mentioned The Hot Spot yet. Fuck me she was amazing. Also while everybody remembers Shannon Elizabeth in American Pie, you can also see lots of her jugs in Dish Dogs.
Other favourites (that definitely involve tits) include :
Reese Witherspoon in Twilight (no, not that one)
Elisabeth Schue in Leaving Las Vegas
Shannen Doherty in Blindfold
And though you might think of them as centerfolds first and actresses second I still need to mention :
Anna Nicole Smith was (ahem) fully utilized for her talents in To The Limit and Skyscraper.
Carmen Electra has a brief titty flash in The Mating Habits of the Earthbound Human and a prolonged softcore sex scene in The Chosen One - it's widely said that this was actually a body double but looking at the size and shape of those lovely big bolt ons, the overall body shape and especially the nipples, I'm genuinely convinced that it actually is Electra, and she just wants us to think it wasn't her.
the Demi Moore lapdance just for you my friend, i hope youtube gonna keep my vids
Haha, if you thought I didn't own a copy of the movie to
wank over watch whenever I want, you thought wrong.