ChiefDelphi (also known as CD, not to be confused with compact disc) is a forum focused around FIRST robotics. Founded in 2001, the userbase consists of students, mentors, and other volunteers interested in high school robotics. Teams will release the design and code for their robots here, ask for advice, improve their strategies, and just generally talk about their love for robots. Though there is plenty of good technical advice to be found there, be advised that the users have a penchant for passive-aggressive arguing, overuse of acronyms and sudden calling for a "water game."
Though the forum is primarily focused on FIRST programs, other student robotics competitions such as VEX Robotics see some discussion as well.
The ChiefDelphi forum is named for the team of the same name, which was given the designation FRC 47. The ChiefDelphi team no longer exists; it merged with another team in 2009 to form FRC 51, the Wings of Fire.
You can access ChiefDelphi at https://www.chiefdelphi.com/