Unless you've been working in an organization whose mission is to supply C++ language tools (and perhaps particularly if you have, come to think of it), you can't help but have noticed that a lot of people who value productive use of time over esoteric language theory have come to the conclusion that the C++ language is hopelessly broken.
Sure, you can do anything in C++... or at least in C++ as specified. How you do it in any particular C++ implementation varies. That's a large part of the justification for libraries like the (excellent) Boost; to tame the wilderness and present a reasonable set of tools that will (almost certainly) work with any (serious) C++ compiler in the wild.
But, as several language professionals have pointed out, very, very few (if any) people know all there is to know about C++; teams using C++ for projects agree (explicitly or otherwise) on what features will and won't be used. Sometimes, as in the case of many FLOSS projects, this is because compiler support has historically been inconsistent across platforms; more generally, it's to keep the codebase within the reasonable capabilities of whatever teammates poke at it over the life of the project.
I've been using C++ for 20+ years now, and I still learn something — and find I've forgotten something — about C++ on every project I embark on. Talking with other, often very senior, developers indicates that this is by no means an isolated experience. Thus, the quest for other language(s) that can be used in all or most of the numerous areas in which C++ code has been written.
If you're on Windows or the Mac, and happy to stay here, this doesn't much affect you beyond "there but for the grace of $DEITY go I." Microsoft folks have C#, designed by Anders Hejlsberg, who previously had designed and led development of the sublime Delphi language. C# essentially takes much of the object model from Delphi, does its best to avoid the various land mines encountered by Java and particularly C++ while remaining quite learnable and conceptually familiar to refugees from those languages.
On the Mac, Objective-C is the language of choice, providing commendably clean, understandable object-oriented wrapping around the nearly-universal C language (which is still fully usable). Objective-C is the reason most Mac-and-other-platform developers cite for their high productivity writing Mac code, along with the excellent Mac OS X system libraries. Objective-C is supported on other platforms, but the community of users on non-Apple or -NeXT systems is relatively small.
Google created the Go language and first made it public in late 2009. The initial design was led by Robert Griesemer, Rob Pike and Ken Thompson. Pike and Thompson are (or should be) very well-known to C and Unix developers; they led the pioneers. Robert Griesemer, currently with Google (obviously), was apparently previously best known in the Oracle data-warehousing community, but a quick search turns up a few interesting-sounding language-related hits prior to Go.
Go is described as "simple, fast, concurrent, safe, fun, " and open source. It doesn't try to be all possible things to all possible users; it has a relatively small, conceptually achievable mission, and leaves everything else to either The Future™ or some to-be-promulgated extension system.
Go is small, young, has a corporate sponsor acting as benevolent dictator with an increasingly active community around the language and, if it doesn't fall into the pits of internal dissension or corporate abandonment (as, say OpenSolaris has), looks set to keep that community happily coding away for some years to come. Given a choice for a greenfield project between C and Go, I can see many possibilities where I'd choose Go. I can see far fewer similar instances where I'd choose C++ over either Go or D.
D is another, rather different, successor language to C++ (which, as the name states, considers itself an improved or "incremented C."). Where Go is starting small and building out as a community effort, D was designed by Walter Bright, another C language luminary. He created the first native compiler for C++ (i.e., the first compiler not producing intermediate C code to be compiled "again"). Unlike Go, D's reference implementation is a revenue-generating commercial product. Unlike Go, D has an extremely rich language and set of core libraries; one can argue that these features make it closer to C++ than to the other languages discussed here. Yet it is that richness, combined with understanding of the (two major competing) leading implementations, that makes D attractive for C++ and Java refugees; they can implement the types of programs as before in a language that is cleaner, better-defined and more understandable than C++ can ever be.
D2, or the second major specification of the D language, appears to have wider community acceptance than D1 did, and we confidently expect multiple solid implementations (including GCC and hopefully LLVM)... if the larger community gets its house in order. I wrote a comment on Leandro Lucarella's blog where I said that "Go could be the kick in the pants that the D people need to finally get their thumb out and start doing things "right" (from an outside-Digital Mars perspective)."
Competition is generally a Good Thing™ for the customers/users of the competing ideas or products. It's arguably had a spotty record in the personal-computing industry (85%+ market share is not a healthy sign), but the successor-to-C++ competition looks to be entering a new and more active phase... from which we can all hope to benefit.