I can't believe that nobody has mentioned GENZOMAN! He does amazing character portraits, mostly, but also monsters and armour and all kinds of goodness. He's done official and professional work for many things, but also concept art and boxart for Infinity: The Game as well as that new wargame called Norsgard, I believe.
Also, everyone, it's against DeviantART's rules to use it as an image hosting site like everyone has been doing. Here are some hard links instead, that also don't stretch the topic.
Also, I highly endorse 343GuiltySpark (now ArtMagix), who was mentioned earlier. Sorry if this is a necro - the last post was a while ago. Since the thread is still open, though, one can only assume that Matt is still looking for concept artists!