Call of Duty
Back in the days I have spent some time playing Call of Duty and even more so creating for Call of Duty. I was allowed membership of a clan by producing the map NLK Bunkerz for them, which is one of the most popular maps I have made (see if you discover the secret room!).
The other major project I worked on was with CoDEmanX programming a BSP decompiler (.bsp ⇒ .map). This didn’t exist for Call of Duty yet, but we never got to release the source code. People were too worried that their maps would get ripped off that we decided only to publish the official maps for CoD1, CoDUO and CoD2.