I am not a fan of GUI-based spellcheckers since it is so easy to miss a misspelling. I much prefer ispell within emacs. In particular, ispell does a great job (compared to any GUI I've seen) of suggesting alternatives, emacs and ispell together do a great job of accepting words for a session across multiple documents, and emacs understands various file formats and doesn't try to tell me that, for example, an HTML tag is not a valid English word. Well, it has long been a common opinion that GUIs are great for beginners but don't particularly reward more experienced users with better productivity. I particularly dislike spell checking within OpenOffice, since when adding to the dictionary one has to always specify which dictionary to save a word to, even if there is only one dictionary. Make the common case fast? I don't think that's a concept OpenOffice developers are familiar with.
So how can one make it less likely to miss misspellings before sending an e-mail, submitting a form, etc? Firefox 3.6 and newer has a nice feature that's a pain to enable, ui.SpellCheckerUnderlineStyle. See http://kb.mozillazine.org/Ui.SpellCheckerUnderlineStyle. I particularly like option 4, which places a double line under each misspelled word. But the article just cited does not give explicit instructions for enabling the feature.
1) Open about:config
2) Right click in the list of preferences and select new.
3) For the new preference name, use ui.SpellCheckerUnderlineStyle.
4) For the type, use integer.
5) For the value, use your preferred value from the MozillaZine article. 5 is the default, a wavy red line. My preference is 4, a double line.