Spelljax, as the name suggests, is an online tool that lets you check for spelling errors in English and some other popular languages. The tool check only the spelling mistakes, not the grammar of your text.
You can either directly type the text in the web browser or copy-paste from another application that doesn't inbuilt spell checking capabilities like notepad or internet clipboards.
The spell checker is available at SpellJax.com.
SpellJax spelling checker functionality is pretty similar to what you have seen in GMail or even Firefox. The feature I liked most is that you cannot edit or type text while you are in the spell checking mode. Thanks Jan.