I love the fonts, colours and layouts for this outlook mockup. If only more email clients looked this good. Designer: Frantisek Kusovsky Source: Dribbble  

Outlook Email Sidebar

A clean no-nonsense UI with a good colour palette. Designer: Matthew Daniels Source: Design Kindle

Sleek UI Elements

This is a great clean UI kit which is available to download from 365psd. Designer: Sebastien Gabriel Source: 365 PSD  

Cloudy UI Kit

This is an excellent free UI for designers which could be used for a range of projects. Designer: Designmodo Source: Designmodo

Pandora Full UI

Source: http://lewisbell.deviantart.com/art/Web-Elements-PSD-1-202286356

Web Elements