The camps for and against auto-correction are clearly divided. Either you like the extra assistance, or it's nothing short of annoying. I fall into the latter group — seeing a red line under my mistakes is guidance enough — but if you prefer blinding typing away with little concern for the rules of language — and you use Chrome — an extension called Spell Bee could help you finally break free of your lexical oppressors.
Available from the Chrome web store, Spell Bee should work for any web-based text-input control. There's also a white-list for sites that should be excluded from its influence and a shortcut feature to paste in common strings, such as signatures and email addresses.
If you need to tweak its dictionary or settings, just hit up the extension's options page. A search field lets you hunt down specific words and phrases, so customising it to suit your tastes is a straightforward affair.