It's kind of funny to walk in on a coworker playing a game. I'm won't mention the names here to protect the innocent, but the following three transgressions come to mind.

• A friend, long ago, was playing a game one late afternoon when his manager angrily walked in and threw a resume (sorry, too lazy to enter the funky correct characters on that word) on his desk and walked out. My friend had forgotten he'd had an interview that day. After that he resolved to be a better worker and not mess around at work, and that has stuck ever since.
  
• I used to sit across from a guy who would often play a game later in the work day, probably after his 8+ hours of work that day. He now works for the game group at Microsoft.
  
• Yesterday, I was trying to tell my coworker about Homestar Runner (a web site that is not work-related in any way really), and found that he was already preoccupied with non-work, the new game Rise of Nations.
  

It's pretty common for people to surf the web and watch or listen to funny media on their computer, but at my work at least, there's little game playing that I see. Amazing since we were the company that put Solitaire in our operating system.