Mikhail from the Visual Studio Web Tools Team just published a cool free plug-in for VS 2005 (that he built in his free-time) that provides support for spell-checking HTML and ASP.NET page content within the VS 2005 IDE:
You can download and learn more about it here. Very cool stuff.
Hope this helps,
P.S. Speaking of cool VS 2005 plug-ins, also check-out this CSS Properties Window plug-in that Barry and Ben built (and which I blogged about back in December).