Wallpaper engine has a permanent spot on my computers. I've never been a fan of wallpaper programs because of technical issues and stability but Wallpaper engine seems to keep everything running smooth.
No restrictions on user content.
You'll get everything from art , to video games, to Music Videos with sound to movies and more.
You can also enable the options for "questionable content" meaning revealing clothing to artistic nudity as well as the option for mature content that ranges from nudity to softcore adult videos. I really like one user made music video showing off the Dead or Alive girls to some non-copyrighted music. It's NSFW sure but beautiful and calming.
No restrictions on user content as well as the incredible amount of animated and stylish wallpapers available is why I love Wallpaper engine.