Ginger Free Grammar Checker
I'm using chromium 23.x snapshot now. A cursory trial of Ginger seems to indicate it's not flaky. It picks off grammar errors such as what it be,
and spelling errors using a wavy green underline. There aren't many free grammar checkers around that don't require pasting into a web site. So far I haven't detected any side-effects typing with Ginger active as I have with some other checkers. For these reasons I'd say it's worth a test drive. Especially if you use a chrome based browser as default.
There are also desktop versions of Ginger for non-chrome browsers.
Note that when I first started typing, Ginger didn't seem to do anything even though I deliberately typed in some "dis and dat" verbiage. However once I clicked the toolbar button it seemed to wake up.