Software to auto-tag text, such as blog post or news articles?

I am searching for a software that would do the following:

1. process numerous articles varying from 100-1k words;
2. automatically remove stop words;
3. recommend best tags based on remaining words, i.e. this article is all about "car, transmission, japan".

To me this is sort of like Google's internal keyword summary tools. Any i开发者_高级运维dea? Will an open source full-text search tool like Ferret be a viable option? But I am totally open to commercial software tools as well.