Operating system icons have gotten progressively more appealing over time but that doesn't mean you're happy with the way your operating system of choice displays things. Iconizer catalogues the swanky icons of multiple operating systems and more for your use.
At Iconizer you can not only browse through categories like icons for actions, communication, hardware & devices, and other types of customisation, but you can browse for icons by operating system. Love the way a certain icon is used on the Mac? Like the way Aero displays things on Vista and Windows 7? You can dip into the icon sets for each operating system and customise icons from it for personal use. Not only are stock icons included but each category has around a dozen additional icons sets independent of entries like Aero, Vista or Tiger.
Iconizer lets you change the background and foreground colour of icons, make the background transparent, adjust the size, and save the file in a variety of formats so you can not only use it for your actual operating system but also for design projects or on your web site. The only moderate fault with Iconizer is that modifying and downloading icons is a one-by-one affair — if you find that you just like the stock icons in a given category you can't grab all of them at once. The service is free and requires no sign up. Have a favourite place for customising or finding new icons? Let's hear about it in the comments.