When writing for my WordPress blog I have used the WordPress plug-in "After the Deadline". As I am not native English, this helps me correct spelling and grammar errors.
Recently the same tool
is released as an extension for Chrome and Firefox. Using this extension it is possible to use the tool with any form on webpage when it is viewed with Chrome or Firefox.
For example, I used it with the tip and article submission form on The Code Project. The best thing is that it is free for personal use
of the tool shows more information. Here are the Chrome extension
and the Firefox extension