One of the tools that I use daily is the Firefox add-on Firebug. If you’re dealing with web development in any way, Firebug will give you an enormous wealth of information and tools:
- monitor how long it takes for each component of your web page to load;
- edit css and html with a live preview;
- debug javascript errors;
- analyze DOM, and more.
Although this is a plug-in, it has its own environment of extensions to add even more to the fun.