My favourite browser, Camino offers build in spell checker in version 1.1 alpha2, which is quite nippy.

What is not so nippy is, that when I for example load a large document in Google Docs, Camino is not all too happy about it, as it runs through all the text to check my spelling. The consequence is that it beach balls, and becomes unresponsive (at least on my 867Mhz 12" G4). I couldn't find anywhere to disable the spell checker in the settings (I could be blind), but then I remembered about:config, and that it might be possible to change it there.

And tada.wav, it is. The trick is to enter about:config in the Camino address bar, and in the provided 'filter' field, type 'spell'. There should now be two entries visible, and if you doubleclick on the entry entitled layout.SpellcheckDefault, you will be met by a prompt into which you type the value 0. The default setting is 2 (for enabled, have no clue what 1 does). Here's an illustration:

Hit enter, and you have disabled the spell check, which makes Camino behave nice, even when loading large documents in Google Docs. The trade-off is that your spelling goes south, but you can't win all the time.