Ask people to name an open source web browser and most will name Firefox, although Google's Chromium is gaining in popularity. In fact, Linux users have long been spoilt for choice in terms of browsers, Firefox, Chromium, Konqueror and many lighter weight choices. Most of the alternatives were based on Firefox's Gecko rendering engine but now WebKit is gaining in popularity. Midori is a lightweight browser based on the WebKit rendering engine.
Midori uses the GTK toolkit, so it looks like many GNOME apps, but it does not need GNOME. The feature list is good, containing the main options expected of a browser these days like tabs, a configurable search gadget, user styles, cookie management and extensions for the most needed features such as ad blocking, form completion and mouse gestures. By not having a full extension architecture that enables plugins to do everything but backup your hard drive, Midori is a lot leaner than other browsers. Yet the use of WebKit means that site rendering is good and fast, and things like flash and HTML5 videos are supported by default.
With a modern, reasonably powerful machine running KDE or GNOME, you may prefer to stick with Chromium or Firefox, but if you are running lightweight hardware, either an older computer or a netbook, an efficient browser like Midori may be just what you need.