In free indexing, keyphrases are significant terms that appear in the document. Any document in the phrase is a potential keyphrase. The advantage of free indexing is that it can be applied to any document. The disadvantages are poor quality of extracted phrases (compared to controlled indexing) and the indexing is not consistent.
In controlled indexing, keyphrases are chosen from a controlled vocabulary (a dictionary, thesaurus, or a list of terms). It has the advantage that all documents are indexed in a consistent way disregarding their wording. For example, two documents, one about "laptops" and another one about "notebooks", would be indexed with the same term, which is the preferred term in the controlled vocabulary to describe this concept.
Kea-4.1 (ZIP, 6.6 MB) -- controlled indexing only
Kea-4.0 (ZIP, 1 MB) -- controlled indexing for agricultural documents only.
Kea-3.0 (ZIP, 512 KB) -- free indexing only.
The oldest version of Kea is still available for download. It is implemented in Perl and Java (and a little C) for Unix systems. It is not straightforward to install it; you will probably have to know a little about Perl and Java. We strongly recommend you read the README file before you attempt it, so that you know what's in store.
KEA has also been integrated into the NLP workbench GATE (http://gate.ac.uk). Please send queries regarding the KEA plugin for GATE to the GATE support mailing list (http://gate.ac.uk/mail/index.html).
There is a IKMV version of KEA 3.0 (for dotnet/C#) developed by Enrico Lu. It is available on his website: http://enricolu.myweb.hinet.net/.