The Firebug extension for Mozilla Firefox allows the debugging, editing, and modifying of any website's CSS, HTML, DOM, and JavaScript, and provides other web development tools.(from wikipedia)
Firebug is really useful to me in web spider/crawler development. It makes writing regular expression easier & faster as I can put the mouse on specific portion of the web page and get it's code using inspect. Another useful feature for me is I can see all the http requests and responses, which also helps me a lot in writing web spiders.
You can get Firebug from getfirebug.com or download from addons.mozilla.org
Speed up your web development using Firebug!