Snow Leopard has a built-in grammar checker (in Cocoa and other certain apps) that will place green dotted lines under any possible grammatical errors it detects.

• You may have to enable this first, in the program's Edit » Spelling and Grammar » Check Grammar With Spelling. However, accessing the corrections panel using Control-Click » Spelling and Grammar » Show Spelling and Grammar has always seemed cumbersome.

Now simply hovering the mouse over the underlined word pops up a tool tip description of the error.

NOTE The first option in the contextual menu will be the replacement suggestion, but the Spelling and Grammar box will typically have more information. Given you probably want to make the suggested change anyway. Using the contextual menu, you can see and replace; the tool tip merely shows the correction, but you can not do anything with it.