NOTE: This post is a little dated, as it does not make use of features in newer releases.
There is a huge number of C libraries that provide functionality that would be very useful in an application. One way to access this functionality from Tcl is to write a binding using Tcl's C API. This requires fairly detailed knowledge of both C as well as Tcl's API. An alternative is to use a Foreign Function Interface extension for Tcl such as cffi or Ffidl.(read more)