There is a very exciting tool that was launched by Microsoft with Visual Studio 11 Developers Preview. That is Page Inspector.
Web development has always not been a very simple job. Every web developer must have spent lots of time to give a better look to the web page and have worked a lot with HTML syntax.
There are a lots of tools that helped the web developers a lot and became a lingua franca in web development community. Some of these tools are
Browser diagnostics tools
Code Wala 