"Aqua offers a photo-illustrative icon style—it approaches the realism of photography but uses the features of illustrations to convey a lot of information in a small space. Icons can be represented in 512 x 512 pixels to allow ample room for detail. Anti-aliasing makes curves and nonrectilinear lines possible. Alpha channels and translucency allow for complex shading and dimensionality. All of these qualities allow you to create lush, vibrant icons that capture the user’s attention."
That's what Apple says in the Human Interface Guidelines document about icons. Icons are a great way to convey information about the application you're using and give you some eye-candy at the same time. In this roundup I've collected 50+ of the most beautiful, well-designed icons of 3rd party apps for Mac OS X. Do you know more? Feel free to share them in the comments.