WordPress gets a better spellchecker, After the Deadline
After the Deadline finds a lot of mistakes that other spellcheckers won't. If you use "new" instead of "knew," for example, ATD will know whether you've got the right one in context. It can also check for clichés, double-negatives, passive voice, and other problems that generally weaken your writing. ATD also gives you fine-tuned control over those stylistic elements: you can have it stop checking for certain problems, or teach it to ignore your intentional, but technically incorrect, word choices. ATD only works with English right now, but other languages are in the works.