There's a fine line, imo, between underrated and underground - loosely defining underground itself, that is. Just because people don't know of an artist doesn't mean that they're underrated; just that they're, well, not well-known. Underrated means to me that a lot of people know of the artist, but the majority of them don't realise how good they (subjectively) are. Of course there are a huge lot of factors that come into it - the main one which I can think of being the countries concerned; an artist driving us all insane with how overplayed they are in England may only just be poking their nose into America, for example, and will therefore seem underrated in America yet grossly overrated in England.
As for examples of underrated artists? I'll... get back to ya on that one. It'd need a good bit of thinking about.
People love Justin Timberlake and his singles, but I don't think many people know just how good he is. His album FutureSex/LoveSounds, to me, is one of the best-sounding pop albums of recent memory. I really am quite excited to try out his upcoming album, and to see more of dat Timberlake/Timbaland collaboration.