Spellchecker for IE: Switch to Safari, FireFox, or Seamonkey....but if you LIKE having your browser suck at life, here. Go get this.
[You must be registered and logged in to see this link.]Spellchecker for FireFox: Built in to the latest version. Unless you're modding Firefox for open source reproduction, there's no sense in not having the latest version.
Spellchecker for Seamonkey: Built in to all supported versions. If you didn't know about this browser, I'd check it out. It's kinda cool.
Spellchecker for Safari: Place the cursor in a text entry field then click Edit on the menu bar and select Spelling -> Check Spelling as You Type.
A small check mark should appear to the left of the Check Spelling as You Type option, indicating that spell checking is now active. From this point forward, Safari will check any text you enter and alert you to possible errors by adding a dashed red underline to any potentially misspelled words. Press and hold the [CONTROL] key while left-clicking the indicated word. A context menu (similar to the one illustrated below) will open and should present one or more possible corrections; highlight the proper word and Safari will replace it in the text field.
You have to activate it each time you use Safari. Confused? Safari is good for loads of things, but it's a pain to use it's spellchecker on a windows OS. Safari is best used as a back-up to FireFox or Seamonkey. Never use it on it's own.
Spellchecker for Opera: Don't.Fucking.Bother. This browser is crap.