Firefox: As you make the rounds commenting at your favourite blog or composing a lengthy email, avoid misspellings or a bad turn of phrase with After the Deadline's excellent grammar and spellcheck Firefox extension.
This nifty little add-on hides in the background on Firefox until you're ready to use it. The next time you visit a message board, fill out a web form, or leave on comment on your favourite website, just hit F7 on your keyboard before you send your message into the intertubes. The extension checks for spelling and grammatical errors and underlines them in hard-to-miss bold colours. (It seems happy to accept both US and Australian English spellings.)
Optional settings let you exclude words and phrases so you can still interact with your L33t friends without getting called out by spellchecker. You can also tell After the Deadline exactly what grammar and style rules you want proofread, including double negatives, passive voice, redundant phrases and more. Right clicking the highlighted words calls up suggested words and grammatical corrections.
If you're prone to forgetting to proofread comments before sending them, the plug-in will prompt you to do so, though you can disable the option if you find it gets annoying. After the Deadline will also skip proofing at whatever web sites you choose so you can comment in LOLspeak to your heart's content.
After the Deadline's Firefox extension is more robust than Firefox's native spellchecker and customisable to boot, so it's well worth downloading to have on hand. Even if you don't usually worry much about your grammar and spelling in comments, it'll be handy to have on hand the next time you write a five-paragraph impassioned plea on a message board on why Firefly should be resurrected.