http://firefox.com/

Firefox is a fast, simple, sophisticated web browser, which provides a secure alternative to Microsoft’s Internet Explorer. Whilst a great choice for all users, it is particularly useful to web developers, who can use the multitude of add ons (Firebug, Foxy Proxy, Firesizer, Snapper, ColorZilla etc) to aid them in their work. Its high levels of standards conformance also make it a good choice for developing web sites.