It's quite possible that I've blogged about and shared with you this recipe before, but it's so simple and so good that I felt the need to share again (with a better picture, for sure!)
I call this "Poor Man's Shrimp Linguine" only because my mother makes another recipe similar to this which is much more complex and definitely more delicious. This one is still really great though; and it's a lot easier and faster to make.
Check it out at Fine Cooking: http://www.finecooking.com/recipes/linguine-lemon-garlic-shrimp.aspx
The cooking time is very short for the shrimp et al, so I always recommend dropping the pasta before you even start melting the butter. I'd also suggest that while the whole thing may seem too watery after you mix in the pasta water and the mascarpone, just keep stirring it over very low heat for a few minutes and you'll find that the pasta absorbs all the water and the sauce gets to be nice and smooth and creamy.
I also usually increase the amount of pasta to about 3/4lbs instead of 1/2 lbs because then I get more leftovers ;)