Now that Opera has switched to WebKit too, on Linux I found only WebKit or Gecko based browsers. The only other one was KDE/Konqueror's KHTML and on Windows IE's Trident. Are there any other alternatives to WebKit and Gecko on Linux?
There is a Wikipedia article titled Comparison of layout engines (HTML) which may be useful. Aside from Gecko, KHTML, Trident, and WebKit, there is Presto, Amaya, and Links - although they are far from being as widely used as the previously mentioned rendering engines. A more complete list of graphical HTML rendering engines can be found here