Opera is cool because you can tile the windows (tabs) and actually see more than one window at a time. (At least this is true in version 7 for windows) It also has the pop-up blocker and other neato features.
I like Opera because it renders pages faster than Mozilla. It doesn't follow the same standards, it seems, and I've gotten used to the way Mozilla does it. Plus, I use Mozilla's mail client, because of its new bayesian-based Junk Mail filters.
If you're a Mozilla (Phoenix, etc) user -- and want to speed up the render time, go
here. Theres a little preference you can set. I noticed a big speed change. Still not as fast as IE or Opera, but a lot faster than the normal Mozilla build.