After reading over my question and some of your replies, it's clear I was too vague.

I'm looking at something that will (ideally) have tens of thousands of concurrent users each using a good amount of bandwidth and server resources. I'm thinking separate login and application servers, as well as load balancing for the back end.

Is anything in particular well suited for this? The two languages I was initially planning on choosing between were C++ and Java, although obviously I wouldn't be bound to one language for the entire project. Or is there some kind of framework designed with something like that in mind?