Here is a partial list of potentially useful tools.

• Modernizr - "Modernizr is a small and simple JavaScript library that helps you take advantage of emerging web technologies (CSS3, HTML5) while still maintaining a fine level of control over older browsers that may not yet support these new technologies." (pulled from their website)
• HTML5 outliner - this is a tool which shows you the DOM structure created from your HTML5 code.
• Validator.nu - alternate validator (instead of just using the W3C validator)
• VideoJS - open source HTML5 video player with fallback for older versions of IE
• For reference purposes (especially since IE is such a widely used browser), examine the IE Test Center periodically (present emphasis is on IE 9).
• HTML5 BoilerPlate - another good place to get started if you want your HTML5 code to deploy in various browsers today (even IE 6).
• http://simon.html5.org/html5-elements (listing of HTML5 elements and attributes).
• HTML5 accessibility features (and their support in current browsers).

Likewise, these are tools which may help with CSS3.

• Modernizr - "Modernizr is a small and simple JavaScript library that helps you take advantage of emerging web technologies (CSS3, HTML5) while still maintaining a fine level of control over older browsers that may not yet support these new technologies." (pulled from their website)
• CSS3Pie - allows Internet Explorer 6-8 to render some CSS3 properties
• eCSStender - allows for cross browser implementations today
• CSS3 generator - if you want to create a specific declaration
• Selectivizr - provides CSS3 capabilities for IE 6 - 8

Lastly, tools to help with SVG are listed here..

• excanvas.js (if you want to work with the <canvas> element in IE8 or earlier versions, this bit of code is very helpful.

There are many useful sites which I can include here. Obviously, this page is a work in progress.

• Conformance checker at the W3C site. (http://validator.w3.org/)