(WIP) Velo is a vaguely Ruby-inspired programming language which unifies strings with code blocks, and scripts with object classes.
(WIP) toolshelf is a "package manager" which doesn't actually install any files. Instead, it stores the source trees of sundry packages in a single directory, and manages your search paths to include the relevant subdirectories of those trees. [MIT license]
BefOS is an "operating system" in the theme of Befunge. It was originally written in Turbo Assembler in the late 90's, but has at this point been translated to NASM. It does not implement much of what you might expect of a Befunge-based operating system, but it does boot and do some cute things. [Public domain]
(WIP) Robin is a programming language which draws from Scheme (via Pixley), PicoLisp, and Erlang. It uses macros, rather than functions, as a fundamental abstraction, and supports a module system, message-passing concurrent processes, and exceptions. [BSD license]
Shelta is an extremely minimal Forth-like language with barely any semantics; it relies on inline machine code to be considered anywhere near Turing-Complete. In the spirit of compilers for languages such as FALSE and Brainfuck, a Shelta-to-8086 compiler was implemented (with help from Ben Olmstead) in less than 512 bytes of 80286 machine code. What's more, it's also been bootstrapped — that is to say, a Shelta compiler was written in Shelta, which was compiled with the original compiler, and then compiled again with the resulting compiler, producing a wholly self-hosted executable!
Larabee is an assembly-like programming language, with Scheme-like syntax,
TPiS (Total Procedures in Scheme) is a static analyzer, written in Scheme, which can check if given Scheme procedures are total (always terminate, on any input) by checking that they are specified primitive-recursively. What's more, it is written almost entirely in a primitive-recursive style, so it can pass its own checks! [BSD license]
Pixley is a strict subset of R5RS Scheme (or, if you prefer, R4RS Scheme), supporting four datatypes (boolean, cons cell, function, and symbol) and a dozen built-in symbols. The Pixley reference interpreter is highly meta-circular, being written in only 124 lines of Pixley. [BSD license]
Arboretuum is an experimental language based on forest-rewriting, a variant of tree-rewriting in which multiple trees are rewritten simultaneously. The language was intended for specifying compilers, with each tree representing
Treacle is an experimental compiler-definition language based on context rewriting, an expressive variant of term rewriting that generalizes the forest-rewriting used by its predecessor Arboretuum. In context rewriting, a separation is made between names and variables, and patterns may contain holes inside which subpatterns may match at any depth. [BSD license]