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.
It is important to note that pure Tcl libraries (such as anything in tcllib) is thread-safe. However, one should be careful when using certain libraries in tcllib that may require external libraries optionally for speed (such as md5). These should perhaps be checked to either not use the external library when built threaded, or verify that the external library is thread-safe.
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