Firefox 2.0.0.3

Firefox is probably the best web browser to date, with plenty of customizations, plug-ins, and add-ons that increase functionality.

It's used extensively by entities requiring high security. In fact, I work at a government research lab and they have completely disabled Internet Explorer and require the use of Firefox exclusively as the web browser of choice.

However, Firefox is not without problems - there are bugs even in the newest version (2.0.0.3 at the time of this writing). Notable problems include the propensity for memory leaks. Also, it occasionally exhibits weird behaviors after having too many tabs open, such as not registering keyboard typing or the arrow keys for scrolling.

Don't let the bad news let you down, though; it's definitely one of the top browsers.

Official Site:
http://www.mozilla.com/en-US/firefox/

Similar software:
Opera

Open Source
Firefox