emacs (GNU Emacs), a wonderfully customizable text editor.
I've been using this for many years and while I've written some elisp to scratch itches and help with my workflows, I feel I'm very much a novice.
A have a number of posts about Emacs.
calc is pretty wonderful, but I don't use it regularly enough to remember all the interesting things I can do with it. So, here are some notes on things I do regularly-but-not-entirely-enough.
First, calc commands are case-sensitive.
To undo and redo: U and D
To enter hex number 0x1d, write 16#1d. By default, it will show up as decimal on the stack (29).
The key sequences d 2, d 8, d 6, and d 0 select binary, octal, hexadecimal, and decimal as the current display radix, respectively. Numbers can always be entered in any radix, though the current radix is used as a default if you press # without any initial digits. A number entered without a # is always interpreted as decimal.