A collection of programming languages named as a single letter. Non-letter-non-digit characters are allowed as well.
The A+ programming language is an APL descendant, like various other languages on this page, because the APL community loves one-letter names. Arthur Whitney (creator of many APL dialects) created A, then Morgan Stanley extended it into A+.
The B programming language is a predecessor of C and not used anymore these days.
No need to introduce C. If this article interests you, you know C. We could also count C++ and C#, since non-letter-non-digit extensions are allowed.
D is the better C++. This is currently my most favorite language, so anything would be very biased. If you have an opportunity to choose languages, consider this a recommendation to checkout out D!
The E programming language is a quite unique language. It focuses on distributed programming and especially on making that secure through capabilities.
There is also Amiga E which was often just called E as well. Wouter van Oortmerssen intended it as as a game scripting language and describes it "a tremendous success, it became one of the most popular programming languages on the amiga." It is available as Freeware.
F# is relatively well known. Basically, O'Caml ported to .NET.
There is also F, which is a subset of Fortran. It wants to be easier to teach, use, and debug than full Fortran.
F* is an ML-like functional programming language aimed at program verification. The main ongoing use case of F* is building a verified, drop-in replacement for the whole HTTPS stack.
G-code is also called G programming language, so it qualifies. It is a numerical control programming language, primarily used to program CNC machines. It looks assembly-like.
There is a real G programming language inside LabView. This one is a graphical data-flow language.
H is a text-based, weakly-typed language. Not much more is known about it.
I is a J-inspired language, which wants to widen the focus on arrays to more data structures.
K is one of the major APL descendants by Arthur Whitney. It is a commercial product used in banks for finance and trading stuff.
L was a language which gave C syntax to TCL.
L is a sibling of E by HP Labs.
L is a Common Lisp subset.
L is a theoretical language in the book "Computability, Complexity, and Languages: Fundamentals of Theoretical Computer Science".
M# focuses on .NET business applications and websites.
There is this paywalled paper, but I cannot take a look at it.
O is a stack based language with one letter commands. For example, "io" reads a line of input (i) and then prints it (o).
The P programming language is for asynchronous event-driven programming. It has been used to implement and validate the USB device driver stack that ships with Microsoft Windows 8 and Windows Phone.
P′′ is primitive formal language from 1964. It was the first language without GOTO proven Turing-complete. Brainfuck is P′′ plus IO.
P# is a Prolog interpreter for .NET.
Q is a wrapper around K and the kdb+ database to make it more readable.
There is also Q#, "a domain-specific programming language used for expressing quantum algorithms. It is to be used for writing sub-programs that execute on an adjunct quantum processor, under the control of a classical host program and computer."
R is a well known statistical programming language. It is considered on par with commercial tools like SAS.
S is a statistical programming language and R is considered an implementation. Most S code runs in R.
T is a Scheme or Lisp dialect. The last release was in 1984, so it can be considered dead.
The U programming language is a personal project of Rob Upcraft. He wanted a simple C-like language to write his own operating system.
There is a V programming language mentioned in a 1985 summary.
The W was created by Viktor Toth in 2001 to program two vintage handeld computer from HP. It is a very simple language, described as C without keywords, types, and standard library.
X# is a low-level programming language somewhere between x86 assembly and C. It is developed within Cosmos, an open-source operating system toolkit.
X++ is a programming language used in one of Microsoft's enterprise resource planning software products. It is derived from C++ and adds garbage collection and SQL query syntax.
Z notation is a formal specification language standardised as ISO/IEC 13568:2002.
If you are looking for a free name, there is none. However, you can probably overwrite H, I, T, V, or W.
On the other hand, why would you give a language a name impossible to google?