Mikhail Arkhipov has released a cool spell checker for HTML and ASP.Net source pages. It uses the Office spelling engine, so you need Office installed, but it's really nice to keep stupid typos out of public-facing pages. Details here: Spell Checker for HTML and ASP.NET pages
If you combine this with WholeTomato Software's Visual Assist X, you get coverage of almost everything in your source. Visual Assist X does spell-checking of comments and quoted strings in source. If you haven't used Visual Assist yet, it's really an awesome tool. I've used it since the 1.0 days and completely forgot how hideous the default Visual Studio experience is without it until VS2005 betas started. For a brief time, I didn't have a corresponding beta from Whole Tomato and I was miserable. You get much better Intellisense, partial and embedded matching of symbols, more extensive syntax-coloring, multiple clipboard rings and tons of other really nice features, in C/C++, C#, ASP, VB [VS2003 and VS2005], and C/C++ [VC 6.0].