I was looking for an online grammar checker like Duden for german, is there anything similar for english, but free?
so far I have found Grammarly, but it is quite expensive.
http://www.grammarly.com/

is it good? does anyone use these online instruments? which is the best?

thanks!

as a pleasant side effect, help me improve the language quality of my EF posts

I use Grammarly - it's fairly comprehensive. Otherwise the built in grammar checker in Word is ok.

These guys http://www.grammar-monster.com/ recommend a company/product called whitesmoke.

Link to details: http://www.grammar-monster.com/free_...r_checker.html

Here is a "special offer" from their website: https://buy.whitesmoke.com/scs/index.php?b=75OFF

The offer seems to be for $US 76 for the 2012 full version. Not sure how that compares to the product you mentioned.

Not tried them. Hopefully as an educated native speaker I shouldn't need to But I do recall when grammar checking was on by default in various MS Office editions over the years, that in very many cases it was very proscriptive about application of rules, such as use of 'the passive voice' in a way entirely inconsistent with real-life modern usage.

In point of fact there are no universally-accepted 'rules' about how English is used, but just because there aren't any rules doesn't mean you can get away with breaking them

Edit: Just ran this text through the MS Word grammar check and it's passed without comment, so maybe things have improved a bit since I last used it.

so I suppose you would never buy this helpful little book (subtitle maybe too small for reading "Grammar Rules - Writing with Military Precision")

I'd prefer a programme able to do some style enhancing and alternative suggestions, rather than just grammar and spelling check which - as far as I know, MS Word does. I've never really used MS Word in any language I write in usually because it's too basic.
I'd certainly use it if I had to write in a language I've never properly learned like french.

Is there any spell checker that understands context. An English teacher friend of mine once wrote a fabulous poem for the kids at school- to show that a spell checker can get some very stupid results.

A Little Poem Regarding Computer Spell Checkers...
Eye halve a spelling chequer
It came with my pea sea
It plainly marques four my revue
Miss steaks eye kin knot sea.
Eye strike a key and type a word
And weight four it two say
Weather eye am wrong oar write
It shows me strait a weigh.
As soon as a mist ache is maid
It nose bee fore two long
And eye can put the error rite
Its rare lea ever wrong.
Eye have run this poem threw it
I am shore your pleased two no
Its letter perfect awl the weigh
My chequer tolled me sew.

soo funny
as soon as I have my trial subscription I'll run it through Grammarly or one of th others - I'll let you know the report

Thing is, each word is spelled correctly - it's just the wrong word. Not sure any spellchecker would be able to differentiate

You'd have to have a program familiar with a model of your desired English usage. And for that, I suppose you'd need to mirror writers with styles you'd like to emulate. Perhaps someone somewhere has written a program to emulate a writing style. It's probably easier to have another reader.