IPython
Used by 5 peopleipython.org ↗

“When I'm coding bots and other projects, I would flail uselessly without the amazing REPL provided by IPython.”
thricedotted Interview, Language hacker, Twitter bot master · via Uses This 

“I am a huge fan of ipython for doing Python hacking at the command-line, and am super excited for the next big release.”
Drew Conway Interview, Researcher (terrorism, maths, stats, machine learning) · via Uses This