Opera has tabs, mouse gestures, built-in email, a unique look, pop-up blocking, and is free with an ad banner next to the address bar. A paid version has no ad banner.
(Mouse gestures, btw, are commands you can make simply by moving the mouse -- ie, sweeping the mouse left goes back, sweeping right goes forward, going up and down reloads, down and to the left closes the tab, etc.) It's known for speed but has some problems rendering some pages with bad html, which were only checked on IE. It was demonstrated once that some microsoft pages specifically check for opera and give it a worse version of the page
Firefox also has tabs, and pop-up blocking, but by default no mouse gestures, and no built-in email. Its benefit is that it's extremely fast, light, and configurable. You can get extensions to add features such as mouse gestures, only showing annoying flash animations if you click to start them, pie menus (that radiate out from your mouse instead of going down to the right, allowing faster choosing) and other fancy stuff. There are hundreds of these extensions, and skins, letting you do just about anything with it. You can easily unblock pop-ups for particular sites, and even block banner ads from certain servers. Firefox seems to have fewer problems than Opera rendering pages made with bad Microsoft html -- I've never had a problem with it.
Firefox doesn't include email but its companion program, Thunderbird, does it VERY well, with excellent spam filters that learn from your marking stuff as spam or not-spam.
Oh yeah, it also has a google searchbar by the address bar, which is really useful.
IE has no tabs, no pop-up blocking without special software, no mouse gestures, and no e-mail. It has more security holes than any other browser and is the vector for many viruses. It can't display pngs correctly (pngs being the new graphics standard to replace gifs). However, it has the benefit of being the standard, and most pages are coded for IE and checked against IE. It's also integrated with the windows file browser, making it the most convenient to access.
Also, IE is only available for Windows (since it was killed on the Mac,) while Opera and Firefox are available on Windows, Linux, and Mac. Opera is also available on OS/2, Solaris, FreeBSD, and QNX, whatever that is.