Figured I'd make a list of all songs from my favourite anime of all time.

This list is for reference only, and contains NO download links. Whether you decide to search for the song yourself and download it or not is totally up to you.

The songs will be listed in order of appearance. In the case of the OP and ED songs, they'll be listed in the form A/B where A is the A-side song and B is the B-side song.
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K-ON!

Cagayake!GIRLS/Happy!? Sorry!!
Don't Say "Lazy"/Sweet Bitter Beauty Song
Tsubasa wo Kudasai%
Fuwa Fuwa Time
Watashi no Koi wa Hocchikisu (Live Version)
Curry Nochi Rice
Cagayake!GIRLS (5-Nin Version)#
Fudepen ~Ballpen~
Fuwa Fuwa Time (Yui Version)
Watashi no Koi wa Hocchikisu (Studio Version)$

% Mio, Ritsu and Tsumugi play this in ep 1 to convince Yui to join them. The full version of the song is a sped-up version. By the way, this song is a cover of a popular song.
# This means that it's a 5-person version of the OP. In other words, Azusa joins the original 4 for this version of the song.
$ The difference between this and the Live Version is that Mio starts off with the first half of the first verse before Yui starts singing in the Live Version. In the Studio Version however, Yui sings the first verse and chorus while Mio sings the second verse and chorus.

K-ON! Character Image Series

Hirasawa Yui
Giita ni Kubittake
Sunday Siesta
Let's Go! (Yui Version)

Akiyama Mio
Heart Goes Boom!!
Hello Little Girl
Let's Go! (Mio Version)

Tainaka Ritsu
Girly Storm Shissou Stick
Mezase Happy 100%
Let's Go! (Ritsu Version)

Kotobuki Tsumugi
Dear My Keys ~Kenban no Mahou~
Hummingbird
Let's Go! (Tsumugi Version)

Nakano Azusa
Jajauma Way To Go
Watashi wa Watashi no Michi wo Iku
Let's Go! (Azusa Version)

Hirasawa Ui
Lovely Sister LOVE
Oui! Ai Kotoba

Manabe Nodoka
Coolly Hotty Tension HI!
Prologue

DEATH DEVIL (Yamanaka Sawako)
Maddy Candy
Hell The World

K-ON!!
Go! Go! Maniac/Genius...!?
Listen!!/Our MAGIC
Amefuri
Pure Pure Heart
Usagi to Kame
Fudepen ~Ballpen~ (YuiAzu Mix)*
Love
Utauyo!! MIRACLE/KIRA KIRA Days
No, Thank You!!/Girls in Wonderland
Gohan wa Okazu
U&I
Tenshi ni Fureta yo!`

*So far, there's no official recording for this. It appears only in episode 9 and does not exist outside the anime.
`I'm guessing this translates to "We Touched an Angel!" (I can't find "Fureta", but I managed to find "Fureru": to touch). This song was written and performed specially for Azusa by Yui, Mio, Ritsu and Mugi in Ep 24. Should be released very soon; there's news of a "Houkago Tea Time II" album in the pipeline.

K-ON!! (Other songs)
GENOM [Performed by DEATH DEVIL, in the album "Love"]
Sakuragaoka Joshi Koutougakkou Kouka (Rock Version)^

^This translates to Sakuragaoka Girls' High School Anthem (Rock Version). Yes, they made a rock version of their school song. =.=

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There's news of a second set of Character Image albums being released. This list will be updated when they've been released.

Note: I originally wanted to put in the video links, but it turned out to take up too much space so...search for them yourself on YouTube! :x