With GEOS 3.2 released in 2009, it's nice to have GEOS 3.3.0 now released. This link also provide details on the JTS features missing in GEOS. Here's the full change list for GEOS 3.3.0. In short, in addition to the usual bug fixes, "this release introduces a fair amount of new C-API interfaces and a brand new PHP binding."
A reminder of what GEOS is: "GEOS (Geometry Engine - Open Source) is a C++ port of the Java Topology Suite (JTS). As such, it aims to contain the complete functionality of JTS in C++. This includes all the OpenGIS Simple Features for SQL spatial predicate functions and spatial operators, as well as specific JTS enhanced topology functions. GEOS is available under the terms of GNU Lesser General Public License (LGPL)."