What is your favourite IDE?

There is a recent post about the new JetBrains Fleet IDE, but of course it is sadly not open source.

What is your favourite IDE? (Or compiler, language, framework, anything: just share your experience and knowledge)

I'll start:

• PHP: Geany
• Python: Thonny
• Pascal/Delphi: Lazarus
• C/C++: wxDev-C++, and Code::Blocks
• C#: MonoDevelop, (before it was SharpDevelop)
• Go: LiteIDE X