Is there a jQuery-like CSS/HTML selector that can be used in C#?

Update 10/18/2012

CsQuery is now in release 1.3. The latest release incorporates a C# port of the validator.nu HTML5 parser. As a result CsQuery will now produce a DOM that uses the HTML5 spec for invalid markup handling and is completely standards compliant.

Original Answer

Old question but new answer. I've recently released version 1.1 of CsQuery, a jQuery port for .NET 4 written in C# that I've been working on for about a year. Also on NuGet as "CsQuery"

The current release implements all CSS2 & CSS3 selectors, all jQuery extensions, and all jQuery DOM manipulation methods. It's got extensive test coverage including all the tests from jQuery and sizzle (the jQuery CSS selection engine). I've also included some performance tests for direct comparisons with Fizzler; for the most part CsQuery dramatically outperforms it. The exception is actually loading the HTML in the first place where Fizzler is faster; I assume this is because fizzler doesn't build an index. You get that time back after your first selection, though.

There's documentation on the github site, but at a basic level it works like this:

Create from a string of HTML

CQ dom = CQ.Create(htmlString);

Load synchronously from the web

CQ dom = CQ.CreateFromUrl("http://www.jquery.com");

Load asynchronously (non-blocking)

CQ.CreateFromUrlAsync("http://www.jquery.com", responseSuccess => {
    Dom = response.Dom;        
}, responseFail => {
    ..
});

Run selectors & do jQuery stuff

var childSpans = dom["div > span"];
childSpans.AddClass("myclass");

the CQ object is like thejQuery object. The property indexer used above is the default method (like $(...).

Output:

string html = dom.Render();

You should definitely see @jamietre's CsQuery. Check out his answer to this question!

Fizzler and Sharp-Query provide similar functionality, but the projects seem to be abandoned.

Not quite jQuery like, but this may help: http://www.codeplex.com/htmlagilitypack

For XML you might use XPath...