Mikhail Arkhipov has written a spell checker for HTML and ASP.NET Pages. Sweet. You do need Office 2003 installed to use it.
Spell checker works in Source view, it is able to extract text from markup elements and use Office 2003 spell checker to check the text. Spell checker is able to handle entities to some extent (they are considered whitespace for now). It is also able to spell check values of attributes that typically contain human readable text. You can customize spell checker behavior by editing an XML file. Since the add-on uses Office spelling engine, you must have Office 2003 or at least Word 2003 installed. Spell checker uses active Office dictionary. Therefore if page is in Japanese, but primary dictionary set in Office is in English, spell checking will be done in English. Current version does not merge words split by tags, such as <b>S</b>ymbol. I am planning to add this functionality in a future version. Right click on the misspelled word does not bring suggestions, double-click does. This is because right-click in VS 2005 HTML editor is hardcoded for the context menu and cannot be overridden.
Important note: add-on does not work with Visual Web Developer Express as Express editions are not extensible.