GUIdebook is "a website dedicated to preserving and showcasing Graphical User Interfaces, as well as various materials related to them.”
1. Screenshots. This site has an amazing repository of screenshots of all the main applications from pretty much all the OS that's ever been introduced, all saved in 24-bit PNGs (aka lossless) quality.
2. Interface Elements. Additionally, you can compare icons of application components, drives and devices, documents and folders, as well as mouse pointers across different OS environments.
3. Sound. Fairly interesting, and is often missing in other GUI repositories is the collection of sounds in WAV (lossless) format, which is a very effective means of user feedback in interface designs.
4. Applications. The site also include application splash screens, and an applications section that detail every aspect of iTunes and Photoshop during their entire history.
5. Additional resources. It even has repositories for ads, videos, articles, books and tutorials of everything known to human history about GUIs.
More information about specific GUI families can be found on the site's site map.