I've been doing a lot of debuging at work using GDB. There are a bunch of useful commands among GDB's huge command list. Here's a quick list of the one's I've found useful:

• make – run make. It can take arguments just like on the command line.
  
• enter – do the same command again.
  
• n (next) – move to the next source line in the file.
  
• s (step) – step into the function.
  
• finish – step out of the current function.
  
• break filename:line number – set a breakpoint at a particular line.
  
• clear – clear the breakpoint you are stopped on.
  
• bt (backtrace) – show the current stack.
  
• frame n – go to the nth stack frame.
  
• info threads – list the current threads.
  
• thread n – look at the nth thread.
  
• print something – print a variable's value, or evaluate something (such as print sizeof(foo)
  
• info locals – print all local variables.
  
• info args – print all of the arguments to the current function as they are now (as opposed to as they were at the top of the function).
  
• call function – call a function.
  
• whatis something – print the type of a thing.
  
• x – just look at help x. This does memory dumps in many formats.