Box2D is famous for using prefixes instead of namespaces
Qt does use Q prefixes at times, not sure if it uses namespaces, but it's a very backwards compatible library so it's understandable.
Erin Catto also uses argument that he supports compilers that don't support namespaces (?!) and explicitly changed all cxxx includes to xxx.h few months ago (back to C from c++ compatibility ones, probably since they use namespaces, the commit had simply stated the fact, not given a reason), also he uses raw memory blocks instead of new (but that's to also allow plugging in other memory allocators). There is also a bug where copying ChainShape(it's that I think, I'm sure one class does that, might not be ChainShape but I'm fairly sure) would cause double free because only pointer gets copied - violated rule of zero/five/three + manual memory management. It's like ultimate bane of your existence, Nexus.
I don't know what to think about that. It's good physics library but the mentality of 'c++ is actually C' is very weird to me personally, especially since I am one of people who really really love C being nearly subset of c++.
I personally like namespaces, and they help with the fact that code completion starts looking only inside namespace when typing it's name followed by ::. They also make code clear and allow me to see easier which 'module' the functionality is coming from at a glance, :: really separates out the namespace name in monospaced fonts. Netbeans has no problem running GDB and listing these classes, it shows the type of class in namespace correctly as 'namespace::ClassName'.