The purpose of this page is to catalog Tcl extensions believed to be safe for use in multi-threaded Tcl applications, along with the version numbers in which thread-safety was added. This list might change as the definition of "thread-safe" evolves in the Tcl community.
Here is a list of thread-safe and not-safe libraries, at the bottom of the page.
http://httpd.apache.org/docs-2.0/developer/thread_safety.html
Well, let me be the first to define "thread-safe":
thread safe code does not fail with a segfault two times,
then once with a bus error, then run for a week followed
by a hardware lockup.
-PSE