I don't think the OP is talking about lisps and stutters.
We've all seen the family out to dinner with mum and dad staring into space and the kids totally absorbed by their Nintendos. The prevalence of modern technology has created massive problems in the development of language skills in kids because it has made it so easy for them to avoid conversation.
It's a skill that has to be practiced just like everything else.
It becomes a vicious cycle as the child grows older - they know their speech isn't good, so they continue to avoid situations where it is necessary.
I've heard of otherwise normally intelligent teenagers who cannot express frighteningly simple things like "I like the way she looks in that dress" without a lot of effort. They speak like you would expect someone to speak after learning a foreign language for about three weeks - they have to think about the words and the order of the words, and they make stuff up that sounds plausible to cover the fact they know they are getting it wrong.
Computer games are part of the problem and I don't think they can be more than a minor part of the solution as theses kids need to learn the visual aspects of communication as well - body language and facial expressions. These people need face to face interaction that involves cooperative problem solving to encourage them to talk.
Reasonably complex board games are probably good - games like Risk maybe? I also think that something like D&D would probably be great for people with this sort of problem.