Tk Drag and Drop is an extension for Tcl/Tk to add native drag and drop capabilities to the Tk toolkit. It is designed for Tk versions 8.3.3 and higher. Native drag and drop are supported under Microsoft Windows (XP, Vista, 7), Mac OS X Leopard (Cocoa framework), and Linux (XDND, drag support missing).
Ellogon is a multi-lingual, cross-platform, general-purpose language engineering environment, developed in order to aid both researchers who are doing research in computational linguistics, as well as companies who produce and deliver language engineering systems. As a language engineering platform, it offers an extensive set of facilities, including tools for processing and visualising textual/HTML/XML data and associated linguistic information, support for lexical resources (like creating and embedding lexicons), tools for creating annotated corpora, accessing databases, comparing annotated data, or transforming linguistic information into vectors for use with various machine learning algorithms.
NLTK is interesting, but only as a starting point :-)
While this article is an interesting reading, I think that some more alternatives must be presented. Yes, NLTK is a very interesting toolkit, especially when it comes to parsing, as a large number of parsers are included. However, a few more alternatives should be presented. First of all, when it comes to language processing, the Tcl scripting language should be also considered. It has the most mature unicode support from all scripting languages (and I think that python's unicode support was based initially on Tcl's) and the only language to my knowledge that has full unicode support in regular expressions. But as the choise of language is also a matter of personal taste, I want to point out that there is a platform specialised for NLP called Ellogon (http://www.ellogon.org) which offers the basis for processing components that can scale to really large corpora, and allows component development in C++,Tcl,Java,Python & Perl. This means that you can have components in various languages that can cooperate, and communicate with each other.