Are you into classical music by any chance? If so, then you should check out Nodame Cantabile. It's a good anime series dedicated to the world of orchestrated music. They play music from many different composers such as Beethoven, Brahms, Mozart, Chopin, Dvorak, Bach, and many others.

I would like to second Bigblah's recommendation of the Choro Club and just the Aria OST in general. Its very relaxing and its the perfect soundtrack for a calm and quiet day.

Since i love DN Angel also ...
 

Here are my picks

Ghost hunt - dark creepy but pretty. Silent Christmas is different and one of the greatest song. Couple that with the Silent Christmas episodes (must watch), you might have a new favorite. OP and ED are really different

Shion no Ou - nice orchestra and piano pieces. "Shoubu Dokoro", "Destiny" are a few of the many. Nice drama anime.

Trinity Blood - Base only on "Tenderness", "Broken wings", "Albion", "Resolution" it already worth it to listen to the CD. Orchestral.

tactic - I believe it is OST 1 that I loved.
X - TV OST - Don't try the movie, you'll regret it. TV is the one. There are other good songs too. Not just Sadame.
Jubei Chan (2) - however 1 has "Omoi". Nice light, and energetic sometime. Very good fighting scenes.

Tsubasa Chronicle - Tons of pretty BGMs.
Naruto Movie - The big Advanture of Princess Fuun" - Surprisingly a great CD. Movie it self it the better one out of the three that are out.
Naruto OSTs - OST 1 is fun, but it does not contain "Grief and Sorrow" however it contains "Sadness and Sorrow". OST 3 has "Grief and Sorrow". I believe OST 1 is during Wave Country Arch and beginning of Chuunin Exam. When it was still a good time. OST 3 is right before and right at the beginning of the filler episode. Yuck!

Tenchi Muyo - Movie 1 - great OST. Dark a lot of time but good. The best movie out of all the Tenchi Muyo.

City hunter - The secret service - Caution: Amazingly good CD to hear during cool rain. Not when the sun shine or if it' s hot. Should be hear as a whole CD, not individualy. Individualy, only track# 11 is good.
Really a strange CD phenomena. A so and so City hunter special.

Gundam Seed - "Senjou no Ketsui", "Justice and Freedom" are just examples of ones of the hidden gems that normaly buried under "Mizu no Akashi" and all the nichol's piano and the OPs or Eds or inserts.

Sailor Moon - If you have time to dig, you' ll fine tons of gems. I had to dig through all the 10 CDs from the Memorial CD box, and they still miss 1 pretty song "Bridging the loneliness"!! As annoying as the anime is, I can not make myself not watching Sailor Moon.

Matentai Loki Ragnarok - OST 1. "Mama" is just an example from OST 1. "Yasashii Omoide" is another example.


Additional:
Vampire Knight, Rescue Wing (Suprisingly good OST), Please Save my Earth (anime sucks though), Starship Operator (cool anime, love the end) are others that I could think of.

A lot of my favorite composers has already been stated earlier. I'd like to add one more (since I believe he wasn't mentioned yet? I might be wrong..)

Sagisu Shiro - He did Neon Genesis Evangelion's soundtracks. I specifically liked the piano pieces such as "Rei I" and "Rei II". He also did Kareshi Kanojo no Jijou. There are lots of piano solos here too--"Arima Souichiro I" and "Ichigo Ichie" are only two of the nice piano solos here.

I think Karekano's the more livelier (happier?) of the two, but it also has its emotional pieces as well. It uses a simplistic approach, making you understand why such a piece is being played at a certain scene easily.

NGE, on the other hand, has many layers to it. It somehow narrates the story of the person (or event) it is themed to. It has that grand/extravagant feel in some tracks, especially the orchestral ones.

On a side note, he composed Bleach's soundtrack too.

Kaoru Wada is a good composer. His D.Gray-Man and Kishin Heidan are great orchestral music.

Kenji Kawai and Koh Otani were alredy mentioned earlier. They are wery interesting authors.