I discovered three new tools today that I am definitely using for my next project.

1. http://html5please.us: A mistake most web developers make, especially when making websites geared toward modern browsers, is blindly using all HTML5/CSS3 features possible. Some of these are still experimental and need to be avoided or used with fallbacks. This site is the perfect primer/reference for such decisions:
2. http://caniuse.com: If (and you’d be brave if you aren’t) use Google Analytics, you can determine the browser share of your users and determine what features you want to use and what should be used with fallbacks:
3. Opera Dragonfly Style Profiler (http://my.opera.com/dragonfly/blog/style-profiler-preview): CSS DOM selection is often neglected when performing website optimizations. This can have a significant impact on your site’s speed. Opera’s Dragonfly can help you optimize CSS selector matching for optimal performance:

Do you know of any others? Please share :-)