When designing a website, if you don't prefer to create everything from scratch, having ready-to-use design elements simplify the process so much.
Besides the ease they offer, such high-quality elements also beautifies the websites being created.
If you are looking for such UI sets, DesignModo is probably the best place to find them as they have a bunch of professionaly-built PSD UI packs.
These packs are all full-layered PSD and includes lots of design elements from buttons to menus, sliders, navigation solutions, pricing tables, charts, icons and much more. And, all having free versions.
Let's take a closer look at the options:
Impressionist UI is the world’s biggest user interface elements pack with 300 elements, 5 styles and all being created PSD shapes.
It has almost every element we can think of and also comes with a 200 icons (both PSD & AI).
And, you can get a taste of this huge set with Impressionist UI Free.
With 200+ elements, all being rasterized PSD layers
There are 2 sets of elements in different colors and two examples of use in web design.
Make sure you also check its free version: Futurico.
Pandora UI for iOS comes with lots of elements (completely focused on mobile interfaces), 5 skins and sample apps for helping mobile developers create good looking apps.
It targets iOS developers, the pack has support for retina display and all elements can be infinitely scaled as they are all Photoshop shapes.
And, as usual, here id the Pandora Free.
This is a relatively smaller (yet still big) UI kit which is focused on both web and mobile interfaces.
It has ~60 elements, all have retina display support
And, every Dark Amber UI purchase is bundled with the popular Unicons icon set.