The unforking of KDE's KHTML and Webkit
There is one major web rendering engine that grew entirely out of the open source world: KHTML is KDE's web renderer which was built from the ground up by the open source community with very little original corporate backing. The code was good and branches were born as a result, the best known being Webkit. Now, after years of split, KHTML and Webkit are coming together once again.
One of the first projects to pick up KHTML for use outside of KDE was AtheOS (now known as Syllable), which abstracted KHTML to produce their own web browser, known as Abrowse.