Keyword extraction software [closed]

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I am looking for a piece of software that, given some text, will extract the "meaningful" keywords. For example, in the sentence:

StackExchange is an online community.

I want "StackExchange" and "online community" to be singled out as semantically-meaningful keywords, but not "is" or "an".

A bit of research led to Alchemy API, which does the job well. Are there other solutions out there, especially open source ones?

Answer 1

There is TexLexan, which is open source. This page also has good list of open source and commercial options.

Answer 2

It's a bit old, but I had really good luck building my own tools on top of MontyLingua.