I used the lambda syntax and it worked fine:

Html.ActionLink<HomeController>(h=>h.Index(),"Home",new {@class="active"})

using the string-only syntax, it does indeed emit the url with a querystring variable class=active. Smells like a bug to me.

 Side by side source:

<div class=SYN_TXT id=syn_row26 style="PADDING-LEFT: 4em; TEXT-INDENT: -0.5em"><LI></div> <div id=syn_block26> <div class=SYN_ROW> <div class=SYN_TXT id=syn_row27 style="PADDING-LEFT: 5em; TEXT-INDENT: -0.5em"><A class="active" href="http://localhost:2222/Home">Home with lambda</A> </div></div></div> <div class=SYN_ROW> <div class=SYN_TXT id=syn_row28 style="PADDING-LEFT: 4em; TEXT-INDENT: -0.5em"></LI></div></div> <div class=SYN_ROW> <div class=SYN_TXT id=syn_row29 style="PADDING-LEFT: 4em; TEXT-INDENT: -0.5em"><LI></div></div> <div id=syn_block29> <div class=SYN_ROW> <div class=SYN_TXT id=syn_row30 style="PADDING-LEFT: 5em; TEXT-INDENT: -0.5em"><A href="http://localhost:2222/Home?class=active">Home with strings</A> </div></div></div> <div class=SYN_ROW> <div class=SYN_TXT id=syn_row31 style="PADDING-LEFT: 4em; TEXT-INDENT: -0.5em"></LI></div></div>

Paul