There are a few other tools that have not made it past version 0.1 and don't appear to be actively being worked on.
Tools such as diction check stylistic issues (I am interested in finding outright bugs), while aspell only checks that words are spelt correctly.
If you are interested in writing a grammar checking tool then you might decide to try and parse sentences. The GPL'd link grammar parser from CMU has been successfully used in a number of projects.
You will also need some information on English usage, some of which can be found here and here.
Those with a general interest in natural language processing can find information on related work here.
Grammatical mistakes found during the production of all versions of the book "The New C Standard" can be found here.
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