Paul Curran, Marketing Manager of Ginger Software, sent me an email about Ginger, their free grammar checker (or proofreader), which "offers both ESL (English as a Second Language) and native English speakers the ability to correct spelling and grammar mistakes with a single-click".
Normally, you have to download Ginger to use it, but you can try it out on a short text at the bottom of this page on their website.
I'm in the middle of correcting student exams at the moment, so I thought I would try Ginger by getting it to correct a sentence containing several mistakes. The original sentence was as follows:
"Today in our global word, we have many company who makes adds, but are they adapt for each person?"
The 'corrected' version was:
"Today in our global world, we have many companies who make adds, but are they adapting for each person?"
So, Ginger corrected the spelling mistake 'word' and the grammar mistakes in 'many company who makes', but failed to correct 'adds' (ads) and mistakenly changed 'adapt' to 'adapting' (it should be 'adapted'). Grammar purists would probably argue that it should be companies that (or which?), because grammar books tell us that "who" is for a person. However, a Google search for "companies who" returns over 17m results, so I think we can consider that usage as fairly standard.
The conclusion is that, as long as you don't expect Ginger to correct all of your mistakes, it can be a useful tool in helping you proofread a text. However, there's still no substitute for a native speaker.