Pyrex [Pyrex] is the compiler project that Cython was originally based on. Many features and the major design decisions of the Cython language were developed by Greg Ewing as part of that project. Today, Cython supersedes the capabilities of Pyrex by providing a substantially higher compatibility with Python code and Python semantics, as well as superior optimisations and better integration with scientific Python extensions like NumPy.
ctypes [ctypes] is a foreign function interface (FFI) for Python. It provides C compatible data types, and allows calling functions in DLLs or shared libraries. It can be used to wrap these libraries in pure Python code. Compared to Cython, it has the major advantage of being in the standard library and being usable directly from Python code, without any additional dependencies. The major drawback is its performance, which suffers from the Python call overhead as all operations must pass through Python code first. Cython, being a compiled language, can avoid much of this overhead by moving more functionality and long-running loops into fast C code.
SWIG [SWIG] is a wrapper code generator. It makes it very easy to parse large API definitions in C/C++ header files, and to generate straight forward wrapper code for a large set of programming languages. As opposed to Cython, however, it is not a programming language itself. Thin wrappers are easy to generate, but the more functionality a wrapper needs to provide, the harder it gets to implement it with SWIG. Cython, on the other hand, makes it very easy to write very elaborate wrapper code specifically for the Python language, and to make it as thin or thick as needed at any given place. Also, there exists third party code for parsing C header files and using it to generate Cython definitions and module skeletons.
ShedSkin [ShedSkin] is an experimental Python-to-C++ compiler. It uses a very powerful whole-module type inference engine to generate a C++ program from (restricted) Python source code. The main drawback is that it has no support for calling the Python/C API for operations it does not support natively, and supports very few of the standard Python modules.