Edit and continue in C# is like magic. Paired with ability to move the execution-pointer or unwind the stack and rerun the function you can write a whole algorithm without ever stopping the debugger. Oops - something went wrong, just change the code, reassign the variable in the watch-window and drag up the execution pointer to previous line. Really powerful!

If only more environments had similar tools. Once you've worked with this other tools feel rather primitive.