What is the latest version of Objective-C?

The "official" Objective-C homepage is The Objective-C Programming Language, as pointed out in the objective-c tag info page.

The latest version of objective C is 2.0. I can't find any single website for the language, but Wikipedia article contains significant information with external links. The Objective-C Programming Language is considered the homepage for this language. You may also be interested in language variants section at Wikipedia. Please note that, though Obj-C 2.0 has garbage collector and iOS use Obj-C 2.0, it does not have GC.

Objective-C 2.0 was released in 2006, and, as of 2016, there is no 3.0. But that doesn't mean it hasn't changed.

You can find a list of the big Objective-C features supported by Xcode here. Sometimes detailed changes can be found in the Xcode release notes here. Other changes, like myobject->isa being replaced by object_getClass(myobject), are not advertised very well and you wind up discovering them through compiler warnings.

Five-year-old question, sure, but the answer constantly changes...