Inserts are sometimes better than the OP and ED themes. You watch a sequence and whoa, you hear a really good song. So far, some of my favourites are:

Akatsuki no Kuruma from GS
Perfect World from Escaflowne
Hatenaki Monologue from Soukyu no Fafner: Right or Left
Winner from FY
Namida no Riyuu from School Days
Your Gravitation from Seto no Hanayome
Lunarian from Seto no Hanayome
Happily Ever After from Tengen Toppa Gurren Lagann

What's yours? Thanks!

I honestly don't know that many image songs. I'm much more familiar with background music than image songs.

~Brothers/Bratja - Michiru Oshima and the Russian choir (Fullmetal Alchemist - and this one's on top of my list because the kids' voices are really touching and the song manages to tell the story of the Elric brothers in under five minutes. Plus, I'm trying to learn it on piano right now)
~Niji no Kanata (Beyond the Rainbow) - Yukino Satsuki (Gun X Sword)
~La Speranza - Hitomi (Gun X Sword)
~Dream Scape - FictionJunction Kaori (Tsubasa RESERVoir CHRoNiCLE)
~Tsubasa - FictionJunction Kaori (Tsubasa)
~Kelas (Let's Dance) - Tayo Awosusi (FMA movie: Conqueror of Shamballa)

Those are some of mine. ^_^ I agree, sometimes insert songs are just as good, if not better, than the OP and ED songs. They need love too.

Tomoe - Iwao Junko (Cardcaptor Sakura)
Yoru no Uta - Iwao Junko (Cardcaptor Sakura)
Atatsuki no Kuruma - FictionJunction Yuuka (Gundam SEED)
Honoo no Tobira - FictionJunction Yuuka (Gundam SEED Destiny)
Gravity - m.o.e.v (Lucky Star)
Take a Shot - Mizuki Nana (Mahou Shoujo Lyrical Nanoha)
Pray - Mizuki Nana (Mahou Shoujo Lyrical Nanoha StrikerS)
Chiisana Hoshi ga Oriru Toki - Kuribayashi Minami (Mai-HiME)
Kaze no Machi he - Fiction Junction Keiko (Tsubasa RESERVoir CHRoNiCLE)
Ensei - Kajiura Yuki (as composer) (Mai-HiME) [Not sure if you consider that as a song...]

True, sometimes insert songs are great too. I particularly love Iwao's songs. There are many background music I like too, mainly those composed by Kajiura Yuki.

Tsubasa - Tsubasa Chronicle
Mizu no Akashi - Gundam Seed
Shizukana Youruni - Gundam Seed
Honoo no Tobira - Seed Destiny

of corse air gear! i love this anime insert songs ! so coool! like: ma babies,love sensation,sku grinder,and many other.
but other great insert song are....Hoono no tobira from gundam seed the song so good

God knows... and Lost my music from Suzumiya Haruhi no Yuutsu

Kanashimi no Mukou he from School Days

You Are My Love- TRC
Amrita- TRC
Jasmin- TRC
Shizukana Yoru Ni- GS
Mizu No Akashi- GS
Fields of Hope- GSD
Mugen no Kaze- Ayashi no Ceres

Honestly, there are many more....^^, too many to list down...^^lol

Most anime I watch don't really have insert songs. The only exceptions I can remember is for Yokohama Kaidashi Kikou and, more importantly, Aria the Animation/Natural.

Among those, my favorite is probably Amefuribana by Makino Yui. (Used in episode 17 of Aria the Natural.) It's a lightly jazzy tune with a laidback feel and soft vocals to it. Some nice chorus too.

'kiri' by MONORAL, from 'Ergo Proxy'.

Those from The Melancholy Of Suzumiy Haruhi n Gundam Seed/Destiny

Shympony-Makino Yui(ARIA the ANIMATION)
Just for You-Round table.feat Nino(ARIA the ANIMATION).
Its just so great!

Hmm, let's see. So many good ones that it's just difficult to choose. Some of the tops would be:

Coccoro (ARIA)
Barcarolle (ARIA)
Akitsuki no Kuruma (GS)
Nowhere (Madlax)
I'm Here (Madlax)
Honoo no Tobira (GSD)
Key of the Twilight (.HS)
The World (.HS)
In the Land of Twilight, Under the Moon (.HS)
Salva Nos (Noir)

There's a VERY good number of other ones, but those are the tops.
However, I have left out ones like Zodiacal Sign, Awake, Saints, Elenore, etc since those don't use real words, and also left out OVA songs like pretty much every vocal track in Bubblegum Crisis or Macross Plus.
The list just starts getting waaaaay too long with all those.

Anything from Air Gear especially Chain

Air Tv = Tori no Uta
Erementar Gerad = Forever..
Elfen Lied = Lilium
OP and ED from Gore Screaming Show

on the French version of the 30 year anniversary DVD box set of Captain Harlock 78... in the last three episodes:

The disco version of the main theme song that you can hear during random parts of the battles. I can't help but dance when I hear it.

Akai Namida from the Shakugan no Shana Movie, God Knows... and Lost my Music from Haruhi.

Quote by FierceDeityAkai Namida from the Shakugan no Shana Movie, God Knows... and Lost my Music from Haruhi.

woo agreed Akai Namida is one of the best insert song out there.. and played during.... *sniff*

some of my fave is Brave Phoenix from Nanoha A's
You're My Love from TRC

I was totally touched after I heard Sun from Seto sing Momotaro-san

Ey, I think I might add a few on my list!

Kataritsugu Koto by Hajime Chitose
After All by Donna Burke
Drastic Soul from SCRYED

Oh, and I found a lot of the songs you guys mentioned and a lot of them are now my favourites!

It's hard to say.

Ensei from Mai Hime
Akatsuki no Kurama from GS
Shinkai no Kodoku from GSD
Stories from Code Geass
Innocent Day from CG
Key of the Twilight (.HS)
You are my love (Tsubasa Chronicles... actually I have found it fabulous in the serie but not really great when I just listen it)

Bratja (FMA);
Mizu no Akashi (GS)
Akatsuki no Kuruma (GS)
Fields of Hope (GSD)
Shinkai no Kodoku (GSD)
Emotion (GSD)
Vestige (GSD)
Brave Heart (Shaman King)

Mizu no Akashi from Gundam Seed
Meteor from Gundam Seed
God Knows from Suzumiya Haruhi no Yuuutsu
A Secret of the Moon from PLANETES
Just for You from ARIA