Whether you're creating a website or mobile app, well designed UI elements are key to a good user experience. Here are a few useful freebies that can help in creating a modern, sleek style.
1. Zulu - For Adobe Fireworks(download)
2. Blueline UI & GUI - For Adobe Fireworks(download)
3. YouTube UI Kit - PSD(download)
4. Cloudy UI Kit - PSD(download)
5. Sleek UI Elements - PSD(download)
An interesting and simple set of UI elements from http://matthewdanielsdesign.com.