When I looking for modern low-resource browser, friend at IRC suggest arora. At that times I already check out Firefox 3 fork called Iceweasel 3. Surprisingly both logo are polar bear, heh but no they are made by different developer.
Arora is probably the first well-know WebKit based browser, meaning it has different rendering engine than any current mainstream browser though interface is strikingly similar to Firefox. More it has lower memory usage than mainstream browser except iceweasel (both tied).
Arora also support flash (and Click to Flash feature similar to Opera). In a test arora's rendering is far slower than firefox especially to Chromium and Opera not slower than IE, like shown in below screenshot:
Page rendering of sf.net and slashdot.org in Arora
Iceweasel and Arora use around 50-60MB of memory in my system to render both sites. While Chrome, Firefox and IE climb up to 90-110MB. All in vanilla configuration. The only browser that match both OSS browser is freeware Opera 10.
Futhermore, when browser minimized only arora and opera that dramatically unloaded from memory become just several MB. IE and Chromium unload its parent process while child (each tab) process remain the same. On the other hand, Mozilla-based don't reduced at all.
Galeon based browser K-Meleon is currently still the most efficient browser for windows.