I upgraded to Lion when it first came out, and the feature I like least is autocorrect. When typing, my Mac routinely corrects my spelling and often inserts incorrect words. How do I turn this feature off?
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Helpful as we find autocorrect on an iOS device – where typing is tricky because of the devices’ smaller keyboards that don’t give physical feedback – we find it distracting on the Mac. We can offer a bit of welcome news for people who don’t care for this feature – not all applications support it and, for those that do, it can be turned off.

To do so, launch System Preferences and select the Language & Text preference pane. There, select the Text tab and disable the Correct Spelling Automatically option. Note that any applications currently open that support autocorrect will continue to correct your spelling and suggest words to you. To prevent them from doing this you must quit the applications and relaunch them. Then autocorrect will be disabled.