Maybe tar? First version is from 1979.

https://en.wikipedia.org/wiki/Tar_(computing)

Although mine was written in 2010.

  % tar --version
  bsdtar 2.8.3 - libarchive 2.8.3

Edit: Actually it's probably telnet which I use to get into lab equipment. First version is from 1973. The version I run was written in 1993.

Oldest as in oldest origin or oldest release?

Irfanview comes to mind as the default image viewer for Windows

Deoxyribonucleic acid

I'm guessing that some of the included BSD tools on my Mac are pretty old, because they're so simple. https://gist.github.com/pete/665971

Does using a recent version of something that's been updated for a while count? Adobe Illustrator's first release was March 1987, making it 32 years old this month; I use it pretty much daily. I'm currently using last year's version.

If not then I think it's Neko. http://splook.com/Software/Neko.html

Lately my Mac has been warning me that Neko will stop working soon once Apple drops support for 32-bit apps and that will be a sad day indeed.

VLC for sure.

>The VideoLan software originated as an academic project in 1996

Warning: nerd at work. The question may be ambiguous. To highlight: compare "what's the oldest software you still use today" to "what's the oldest software you use today". The first one refers to software you have been using over a longish(?) period and the second to software that may be old that you are using even though you may have only begun using it. I hope my pedantry (pedanticism?) isn't too inane...

Command line: basic Unix utilities (cd, anyone?)

GUI, used regularly: BBEdit, 1992

GUI, used infrequently: MS Word, 1983

GUI, not updated in forever: abandonware games played in DosBox

Winamp 5.6, on all my Windows PCs. It's my most missed software on my Mac. I'd even install it in my car if I could...

I have been using the same web browser, in terms of codebase, ideology and heritage, for over a quarter century now.

NCSA Mosaic -> Netscape 2, 3, 4/4.78, 6 -> Firebird -> Firefox -> Waterfox.

I have run other web browsers such as Opera and Vivaldi, but my main browser has always been and will always be a Mozilla product.

Assuming we count recent versions of old-school software mine are:

foobar2000 vim (I was still being breastfed when Bram started)

Probably vim.

I still use xv by John Bradley regularly.. the most recent version 3.10a was released on 29/12/1994 !

https://en.wikipedia.org/wiki/Xv_(software)

GNU grep. If you cheat and count years since the original grep it's pretty old.

My father, a chemistry professor, still uses PegasusMail which has been around since approx the year 1990. He is also good at handling his mail through `pine` program over a terminal.

On the gaming side: Star Control 2: The Ur-Quan Masters. I'll still play Battlezone once in awhile, which goes back to 1980.

Oldest still supported? Probably gotta be GCC.

Oldest unsupported? I have some VB6 tools I use to make tilesets for Sega Genesis homebrews.

winamp 2.7 from 2001 for my mp3s. Still works fine on windows 10

ed, emacs.

Emacs and GHC

ls for showing files.

Xixit!

If only for the music tracks.

GNU coreutils.