How to spell-check any web site using Firefox
Once you've clicked the bookmarklet or copied and pasted the code, Firefox will turn on the spellchecker for the page you're currently on. All the words that are either misspelled or absent from the Firefox dictionary will be highlighted in red. And you can right-click on a word to see suggested spellings or even to "correct" it. Of course, any changes you make won't actually be saved to the web page, and the next time you load the page they'll be gone.
This trick can come in handy for web publishers looking to proofread their pages, or for nitpickers who enjoy pointing out the mistakes we make when we blog in a hurry.