This is one of the most useful features released by Google this year. Google Suggest, which usually auto-completes your query as you type, is now also a spell-checker. Even if Google can't find popular queries that start with the words you've typed, it will still show a "did you mean" entry that corrects your spelling mistakes.
The most impressive thing about this feature is that the spell checker is context-sensitive, so the suggestions are actually relevant. It's likely that Google uses the smart spell checker from Google Wave.
If you type [this is a rlly], Google suggests that [this is a rally] is more appropriate. Most browsers offer the same suggestion. If you type another word and your query is [this is a rlly beautiful], Google shows a different suggestion: [this is a really beautiful].
Type [Why its so important too eat hole grains] in a text field from a web page and your browser won't find any spelling mistakes. Not even Gmail's spell checker can find the mistakes. That's because most applications use dictionaries to find the words that are spelled incorrectly. Google Suggest is smarter because it tries to find if the words make sense in the context of your query.