A package of skins with a "theme" or by a single author

Skins that monitor system information

Clocks and timer skins

Weather skins

RSS, ATOM and other feeds, GMail, Stocks, any information retrieved from the internet

Media controls, music players, video and animated visualizers

Quote of the Day, To-Do List and other text-based skins

Skins with graphics and animations

Skins to open folders and launch applications and websites

Skins that control functions in Windows or Rainmeter

Skins that don't neatly fit into other categories