In my Evolve talk, I had a slide which talked about inspiration, and some good sites to get it from.
I'm not sure if I agree that these are "the 50 most beautiful", nor how they worked out the user experience for apps which are only designs (not built), but in general they are nice with a few outstanding ones.
I love the flat - but not too flat - look of this. This is pretty close to how I want iOS7 to look. Subtle textures, no "leather", information rich but not too dense.
This one mostly caught my eye as we are thinking of doing something in a similar space to this. It's a bit over-flat for my liking, but nice none the less.
Leonie found this one, and I agree with her - it looks stunning. I'm not overly interested in game development, but I love the mix of flat shaded polygons and minimal texture (eg on the ivy). Reminds me a lot of early games like Another World.
I think inspiration is an important thing when working out how you want your app to look and work, and downloading every possible app isn't really possible (or overly affordable). Tools like these make it a lot easier, and give you ideas of what other designers and developers are thinking.
It's especially important when I'm learning design, I think, to take inspiration from others, as I'm still in the "not sure what can be done" stage. I suspect it'll become less important as I get more experienced.