Sleepy.ab are pretty good. They remind me of Radiohead a lot, finding their albums takes a little bit of time. Palette is my favorite album of theirs. I don't know about much other jpop.
Sakamoto Maaya is all I can recommend in the way of easy listening (oh wait, that's the name of one of her albums). This evil, vile, despicable woman is part of the 5 artists I listen to day in, day out... bitch owns my ears, man... but I still haven't found Hotchpotch yet.
If you want, you can try downloading the vocal CD to Aria the Natural. They're all songs that are... really calm and mellow. Eufonius isn't too bad, either. The vocalist, Riya, did the Dango song from Clannad (where I first heard them). So far as I can tell, their album stuff isn't nearly as upbeat as the singles they put out for anime and other such. Apocrypha is a pretty awesome song, though, as are Reflectia and Meg Mell (dunno where the first one's from, but the others are True Tears and Clannad respectively)
An on and off relaxing group is Round Table ft. Nino. Couldn't find any individual stuff from either of them for the life of me, but together they're pretty chill to listen to. My favorite song from the Aria vocal CD, Rainbow, was done by this group.
and... WHY THE FUCK DOES EVERY PERSON ONLY HEAR AME GA FURU?!!?!? SAKAMOTO HAS TO MANY FUCKING BETTER SONGS UGH /rage... 'cause really, she does. Hemisphere, Garden of Everything, Shippo no Uta, Getsuyou no Asa, etc. Well, idk how the lyrics are, but the sound of them all >>>>>> ame ga furu.
Oh, and whoever sings the intro songs to the Ef: a Tale of Two anime series,I think called Euphoric Field, is good (amazing to me, anyway).
edit: to clarify, I only named things I figured qualified for your search. Left stuff like Orange Range and Maximum the Hormone out. But, if you want a good laugh, listening to Maximum the Hormone is not a bad idea. First time i heard them i was like... WTF?!?! Is this Pop Metal?
ef- tale of memories #1: "euphoric field" by Tenmon feat. ELISA (eps 3-9, 11) #2: "euphoric field (Japanese ver.)" by Tenmon feat. ELISA (ep 12) ef - a tale of melodies #01: "ebullient future" by Elisa