Personally I've been listening to a lot of Hanasaku Iroha and Usagi Drop tracks recently. Other than that tracks from K-On appear quite a bit on my playlists. It's hard to really say as most of the time I listen to playlists instead of entire soundtrack CD's so they'll likely have tracks from many different series on them.
My current commute playlist consists of ...
Allegro Cantabile - Suemitsu and the Suemiths (Nodame Cantabile)
Hazy - Sphere (Hanasaku Iroha)
Sweet Drops - Puffy (Usagi Drop)
Hanasaku Iroha - Clammbon (Hanasaku Iroha)
Tenshi ni Fureta Yo! - Houkago Tea Time (K-On!!)
You Raise Me Up (JP Version) - Lena Park (RomeoXJuliet)
Kimi no Shira nai Monogatari - Supercell (Bakemonogatari)
High, High, High - Kasarinchu (Usagi Drop)
Hana no Iro - nano.RIPE (Hanasaku Iroha)
Sakura Mau Kono Yakusoku no ... -
Seioh Gakuen Kōka Band (Manabi Straight)
Ready!! (M@ster version) - 765Pro Allstars (IdolM@ster)
U&I - Houkago Tea Time (K-On!!)
Fuwa Fuwa Time - Houkago Tea Time (K-On!)
Sweet Drops (Acoustic piano version) - (Usagi Drop OST)
For You - Rei Fu (Wandering Son)
You Raise Me Up (English version) - Lena Park (RomeoXJuliett)
Kawaru Mirai - Choucho (Kami-sama no Memo-chō)
A Happy Life - Megumi Hayashibara (Manabi Straight)
Omokage Warp - nano.RIPE (Hanasaku Iroha)
Eago no Riyuu - Meg Rock (Asu no Yoichi!)
Garnet - Hanako Oku (The Girl Who Leapt Through Time)
High, High, High (Acoustic piano version) - (Usagi Drop OST)
Houkago Tea Time - Houkago Tea Time (K-On!!)
And I do have CDs for them
But that is only one of several anime playlists on my iPod.