You can get the most recent pre-pre-pre-pre-prerelease (Note: features missing. Like, pretty much all of 'em.) of both Niwt and Plines (Niwt requires Plines to be installed), at one of the following links:
NOTE: The current iteration of niwt requires that netcat be installed (as "nc"), and supports a -q timeout option. It also requires Perl for a couple things; other than that, any Unix installation ought to have the rest of the dependencies (shell utilities of various sorts).
Niwt’s source code is free and open source software, and is available under the MIT (simple BSD-style) license.
Do the untar, ./configure && make && make install routine, first on plines, and then niwt, and then go read TryingOutNiwt to see what you can do with your new toy.
Alternatively, you can grab the sources from their mercurial (hg) repositories at http://micah.cowan.name/hg/plines/ and http://micah.cowan.name/hg/niwt/. Of course, if you do this, you'll need to be sure you have GNU's autotools installed, and run the ./autogeh.sh script, which, if you're lucky, should generate your configure and Makefile.in files.