Games:

• Sudoku and more Sudoku (Yahoo)
• FreeCell
• Crosswords (Guardian)
  
• Kreuzworträtsel (Die Zeit)
  
• Crack It
  
• Bubble Shooter
  
• Online Logic Games
  
• Store web games

Fun&Info Videos

• TED-Ed
  
• MinuteEarth
• MinutePhysics
• Kurzgesagt ("in a nutshell")

Stars:

• Astronomy Picture of the Day

• HubbleSite NewsCenter

• Sky Map 
• AstroBiology Newsletter

Fun & Nonsense:

• XKCD
• 3D Fractals
  

• Wordle

Other:

• Onlinestreet: Ausgewählte Webseiten, Erfahrungsberichte & Nachschlagewerk
• Less Wrong - a community blog devoted to refining the art of human rationality
• Edge: Conversations on the edge of human knowledge
• wiseGEEK: clear answers for common questions
• Memrise
• Lifehacker
• Wonder Ho To
• David Colquhoun's Improbable Science
• Number Watch

• Photography

• Amnesty International
• LACUNA
• AVEN - Asexual Visibility and Education Network
• MiDoMi (Search for Music Using Your Voice)