Syntax-highlighting textbox for Winforms, ASP.NET, WPF

Perhaps consider CodeMirror items for the web. Obviously this won't work for WPF as you'd asked, but it'll work for any web framework - Webforms, ASP.NET MVC, plain HTML, PHP and others.

CodeMirror is a JavaScript library that can be used to create a relatively pleasant editor interface for code-like content ― computer programs, HTML markup, and similar. If a parser has been written for the language you are editing (see below for a list of supported languages), the code will be coloured, and the editor will help you with indentation.

Parsers for :

• JavaScript
• XML/HTML
• CSS
• HTML mixed-mode
• SPARQL
• HTML+PHP mixed-mode
• Python
• Lua
• Ruby
• SQL
• PLSQL
• diff
• Groovy
• C#
• Scheme
• Java
• XQuery
• OmetaJS
• Freemarker

Implementation:

<textarea  rows="30" cols="120" id="someCode" >
    //some comments
    var foo = "bar";</textarea>

You include 2 .js references, and then this bit of JavaScript replaces your textarea elements with new syntax highlighted elements.

<script type="text/javascript">
    var textarea = document.getElementById('someCode');
    var editor = new MirrorFrame(CodeMirror.replace(textarea), {
        height: "350px",
        content: textarea.value,
        parserfile: ["tokenizejavascript.js", "parsejavascript.js"],
        stylesheet: "css/jscolors.css",
        path: "js/",
        autoMatchParens: true
    });
</script>

The CodeMirror manual helps.

Also consider the WikiPedia entry for Comparison of JavaScript-based source code editors