Quoting Christian Neukirchen <chneukirchen@gmail.com>:
I didn't intent to question LISP 1.5, but the significance it made.
Say, compared to Scheme, LISP 1.5 is not that world-changing, is it?
In the context of *Lisp*, Scheme is absolutely earth-shaking. But in the general
context of programming languages and my personal experience of their history,
Lisp 1.5 was the most profound paradigm shift.
If someone were to come to me today and ask me, "Should I learn Scheme or Common
Lisp?" I'd have a tough choice, but I think Scheme would win -- it's a much
better learning tool than Common Lisp. But two of Scheme's three major
innovations -- lexical scoping and continuations -- made it into Common Lisp.
I thought of Forth too. Forth's biggest feature are the abstractions
it makes possible, but it's biggest failure is that these leak easily.
But it is an example that should be studied by every language
designer.
It's also addictive. Once your mind connects all the dots, once you enter into
the world and conventions of Forth, and start hanging out with other Forthers,
it can be difficult to return to more conventional languages. Fortunately, one
can indeed earn a decent living in Forth should one decide to embrace the Forth
way of life.
APL lives on as J, K and Q. I still wait for the day I get to see the
K source code... it must be an amazing piece of code.
The open source version of "APL" is A+ or aplus, depending on who you talk to.
I'm on their mailing list -- it was very quiet for most of the year but it
seems to be picking up in activity. It's in Debian.
Further languages that changed my way to think about programming:
- Prolog (mainly the different evaluation mechanism)
Apparently there has been a rebirth of activity in Prolog and Prolog-like
languages such as Mercury and the mini-Kanren package built in Scheme. I
thought it was dead. I made a feeble attempt to learn it, but it made my head
hurt. It was way too far away from my "roots" as a numerical practitioner.
- Maude (mixfix syntax, meta-implementation)
Never heard of it.
- Haskell (typing done right?)
There are just enough useful pieces of software written in Haskell (and OCaml,
too, for that matter) that you need to "have it on your hard drive". If those
pieces of software get useful enough, they'll need to be re-written in Java,
C++ or C#. 
- Pico (http://pico.vub.ac.be/, Lua done right?)
I've heard of Lua -- never heard of Pico
I'm sure I forgot something.
Well, the whole ML family, of which OCaml seems to be the only thriving member.