Adam Lambert - he did good in first week album sales and he had a hit single + 2 moderate singles.. but I still feel that he's very underrated in todays music.
Ashlee Simpson - such an amazing, great artist who is spectacular at whatever type of song she does, no idea whatsoever on why she's flopping. :-/
Cee-Lo Green - did great with '**** You', now I want to see the rest of his amazing songs have the same success.
Fefe Dobson - as you said
Mika - a great, unique and talented artist.. don't even think he's had a hit in the US. I think he could possibly pull an Adele here.
MCR - used to be pretty big, now they're steadily falling lower and lower.
Neon Trees - Animal was successful but that's it, '1983' and 'Your Surrender' deserve(d) to have the same success.
OneRepublic - was/is always a great band.
Panic! At The Disco - worthy enough to win the biggest MTV award of the night years ago.. and yet suddenly are ignored by a ton of people? The new album they put out is great.
Sara Barielles - one of the most talented and beautiful voices I've ever heard.. just, flawless.
The Script - didn't like their first album beside the 2 singles they had but their follow-up is one of my favorite albums.
Taio Cruz, Jason Derulo, Avril Lavigne, Timbaland (kinda), Jordin Sparks (kinda), Jay Sean - successful in singles, the total opposite in album sales.
Train - It seems everyone in the world heard 'Drops of Jupiter' by this amazing band back in the late 90's, and yet nobody knew what a 'Train' was when they 'came out' with Hey, Soul Sister. Granted, I didn't either.. but having one smash single and 2 that are pretty unknown (both hit top 40, but they could've gotten higher) is bull when they have such great songs.