Critcl lets you easily embed C code in Tcl.
A (certainly incomplete) list of packages using critcl is:
Readers wishing to make their use of critcl public here are asked to file a ticket or pull request here, against branch gh-pages
More notes are available on the Tclers' Wiki
Slides for "Critcl - Beyond Stubs and Compilers" [ PDF ], [ HTML ] A paper from the Ninth Annual Tcl/Tk conference (2002, Vancouver).
Critcl currently has two major branches, version 2, and version 3.
Only version 3 is under active development. Version 2 effectively reached its end of life in 2011.
See Changes for version 3 for the detailed list of changes per release.
The various downloads available here are: