K-Meleon: It's like Firefox, without the bloat
Because K-Meleon lacks some of the bells and whistles you'd find in Firefox, Internet Explorer, or Opera, the browser is fast. Like really fast. It also doesn't use as much RAM as Firefox, which is known to be something of a memory hog.
That's not to say that K-Meleon is a one-trick pony. The browser is customizable, and you can choose from a variety of skins and themes. And K-Meleon supports tabbed browsing, mouse gestures, pop up blocking, and macros.
K-Meleon is Windows only. But if you're looking for light weight Mac or Linux web browsers that use the same Gecko rendering engine as Firefox, check out Camino or Galeon.