I always loved Spitalfield but I think their albums post "Remember Right Now" explains their lack of huge success. They changed it up a bit and it just didn't really hit. I can't knock them for trying something a bit different, though. Northstars "Pollyanna" is great album that never hit it as well as it could have, but I think they broke up a year after its release so having never followed that up definitely hurt them. The Postal Service isn't necessarily underrated, but I wanted to use this forum to say I really wish they did a follow up to "Give Up"
Now a days I am very much into Born Without Bones and The Hotel Year. Hopefully they continue to put out releases and tour and stuff and people will get behind them.
I know I am really late on this thread but the original poster who mentioned the Living End is right on the money. That band continuely puts out soid records year after year. They are huge in their own country but I saw them here in Philly back in 05 and there were about 300 people there. The band was just happy to play for us and they played fast and loud with no cares given. I really love those dudes.
Moneen was another band that was really underrated. They would always end up as a supporting slot to bands like Saves The Day but I never saw them do a headliner tour. With that said Red Tree was a banger of a record.
There has been some way more underrated bands:
One Dead Three Wounded
All Else Failed