MozillaBrowser: Other GeckoEngine browsers are more lightweight than this original uber-browser-beast.
MozillaFirefox: Originally Phoenix, then Firebird, then Firefox, now rumored to be the next MozillaBrowser. Light-weight, fast, tabbed browsing, active developer and tweaker community.
NetscapeNavigator: Since the late 1990s, Netscape is an alternative browser (to Explorer, that is). Netscape has been Gecko-based since version 6.
Arachne (http://arachne.cz/): "a fullscreen graphical World Wide Web browser, which is currently available for DOS-compatible operating systems and for Linux/SVGAlib platform."
CharonBrowser comes with and runs under the virtual InfernoOs, the entire download is 57MB -- you're likely to have more RAM than that!