A while back I posted a complaint about the lack of native spellchecking in the major browsers, stating-
In the forum and blogging world, it would be beneficial if more tools supported convenient and efficient automatic spell-checking (the fact that no major browser has incorporated native TEXTAREA spell-checking thus far is a travesty. Any of them could have a killer feature if they simply added Word-like squiggly underlining of suspect words, with easy alternative corrections). As it is, many tools have nothing at all, and the few that do often host a ridiculously unintuitive, hacked-in partial solution.
For those who like release-quality software, salvation is at hand: Grab yourself a copy of the just released Firefox 2; an already brilliant browser that now also supports native data entry spellchecking, with little squiggly (or at least dotted) lines under suspect text, and a suggested list of alternatives.
Finally!
Hopefully this improves the quality of spelling on the net.

And to head off the inevitable comments, no, the lack of spellchecking clearly hasn't somehow improved spelling (usually argued under the "it makes you work harder at it, therefore growing spell-muscles" explanation). Given that our brains learn spelling by example, even those of us who care are misled by a constant stream of misspellings. With the addition of finger-wagging little squiggle-lines, perhaps the purity of the language will improve.
It doesn't analyze grammar, pointing out errant uses of too or to, whether it's a possessive apostrophe rather than a contraction, or help in the endless "that isn't ironic!" debate, but at least it's a start.
The best browser gets even better.