Assembly Languages
Nearly as low level as you can go. It's not quite as hard to work with as you would think.
C
Is generally considered to be a mid level language. It literally has the bare minimum features required to write
an operating system. Overall I consider it to be a nice language, though it has some rough edges such as text
macros and poor support for variadic functions. Its large amount of undefined behavior is infamous. I can't
recommend it for a new project unless it's for educational purposes.
C++
Is generally considered to be a mid level language, though it does have reference counting, finalizers, and
better metaprogramming than C. It is known for being a complicated language.
Common Lisp
A high level language where users can define their own keywords. Unfortunately, the most convenient way to write
it is in the ancient Emacs. The ANSI standard is also quite old, so newer features such as concurrency have to
be imported as libraries.
Emacs Lisp
Is similar to Common Lisp, but slow and not as well thought out. It is the scripting language of the Emacs
editor.
LaTeX
A typesetting language. I've found it a hard to work with, but it is excellent for automation and formatting
large amounts of text.
Lua
A small high level language designed to interoperate with C. Known for its size, speed, and array indexing that
starts at 1.
Machine Code
Nearly as low level as you can go. It's not quite as hard to work with as you would think.
Maxima
A computer algebra system. The code is open source.
Microcode
The lowest level language you can use, unless you count hardware description langauges. It is the finite state
machine that implements the instruction set of the CPU. The microcode I have written targeted simple Moore
machines that controlled the clocks and output enables of registers. If you make a mistake in the code, you
could end up shorting out one of the processor busses (assuming this wasn't implemented in programmable logic).
If you want an assembler for this, you will usually have to write your own.
Python
A high level language that focuses on readability. It uses indentation to control program flow, which
periodically causes bugs. It presents itself as a reasonable high level language, but it's quite easy to run
into language limitations, particularly if you attempt to mix procedural and functional styles. Python has a
large number of libraries.
Tcl
Is similar to Emacs Lisp, but not as well thought out. Seriously though, despite its syntax that looks like it
was inspired by C, it feels more like an early lisp that was built around string processing. It is dynamically
scoped and evaluation of arguments is always optional. Since every object is represented by lists of symbols (or
a string if that's how you think of it), strong typing is quite difficult.
Verilog
A hardware description language. Verilog 2008 and later is now known as SystemVerilog.
VHDL
A hardware description language. It has records, unlike Verilog.