Katy Perry's song 'Part Of Me' is fantastic but has been on YouTube for months and will probably never be made an official single, album track or B-Side.

http://www.youtube.com/watch?v=GDTyTlzfpZs

(it's at a different pitch because of copyright reasons)

And Pitbull/Chris Brown's song 'Where Do We Go' is actually AMAZING. Yet it's seemingly just going to forever remain a "YouTube hit".

http://www.youtube.com/watch?v=XPHIr9gGWnw

Leona Lewis - Scene Of The Crime (for the Echo era)

Britney Spears - Baby Boy (for Blackout era)

I remember that song, I used to have it on iTunes (*cough* ), I think I preferred 'Perfect Stranger', though.

Kate Bush - Out Of The Storm
Madonna - Liquid Love
Britney Spears - Little Me
Lady GaGa - No Way & Future Love

Lady Gaga - Dirty Ice cream

Leona Lewis - Perfect Stranger and Alexandra Burke ft. Bruno Mars - Before It Explodes...

I just wish that these were released

Madonna - Supernatural (would have loved it on Like A Prayer)
Madonna - goodbye To Innocence (the Erotica sessions)
Madonna - Like A Flower (Only on Japan pressings of the ROL album)
Britney - Scary (not available on all international albums bar Japan)
Xtina - Little Dreamer (for Bionic)
Friendly Fires - Kiss Of Life (only available for download from a deluxe version of their debut album)

Roxy Music - Pyjamarama
Siouxsie & The Banshees - Fireworks/Israel
Luscious Jackson - Queen of bliss (b side to 'here')
Courtney Love - Violet
David Bowie - John Im Only Dancing (disco mix!)
Holy Ghost! - I will come back
Friendly Fires - Kiss of life (thankyou Mr Pinder!)

anytime mr muggles

it is a class song, wondered why it wasnt on standard pressings??

We're allowed b-sides too, right? If so, then my favourites are:

The Saturdays - Flashback, Crashing Down, Unofficial, Ready To Rise
Girls Aloud - Memory Of You, She, Crazy Fool, I Don't Really Hate You, Hoxton Heroes
Siobhan Donaghy - Don't Take Me Back
Lily Allen - Fag Hag, Kabul Shit
Girls Can't Catch - Love Shy
Diana Vickers - Cold Kiss
Nicola Roberts - Disco Blisters & A Comedown
Sophie Ellis-Bextor - Yes Sir I Can Boogie, Sophia Loren

My favourite demo is Nothing In Common by Rachel Stevens. Its a Richard X-produced song that was cut from her last album. Its amazing: http://www.youtube.com/watch?v=7mYljQrb1PU

Britney Spears - She'll Never Be Me, Scary
The Saturdays - Unofficial, Golden Rules, I Can't Wait, Had It With Today
Roisin Murphy - Sunshine, Pandora, Foolish, Ripples

Florence - Hospital Beds and Swimming (for the Lungs era)

Gadzooks!! I forgot the peerless Roisin Murphy -Pandora!!
Also most Marc Almond B-sides - Kept Boy, King Of Fools,

The Banshees - Sleepwalking too...

Depeche Mode - Get The Balance Right

Not sure really, im a latecomer to their material, but I love their sound, FF's, Holy Ghost! & Cut Copy are my fav bands atmo, and I guess their sounds have certain similarities, HG! & CC have remixed each other. All three have elements of duurty diy disco/funk in them, which is why I'm drawn to them!
Perhaps KOL was recorded after the album. Lots of bands record sporadically if they suddenly have 'the muse'!

Coldplay - Sleeping Sun - love it!
Madonna - Supernatural - comedy track maybe and you'll either love it or hate it but it works for me. Gets into your head.

Janet - Put Your Hands On (only on the Japanese Damita Jo)
Janet - Pops Up (DJ sessions)
Kylie - On The Up (Body Language)
Kylie - At The End Of The Day (KM94 sessions)
Nerina Pallot - Photograph (due to be on a re-issue of her 1st album)

Rihanna - Hatin on the club (from the Good girl era)
Rihanna Feat. Akon - Emergency Room (from the Rated r era)

Florence and the machine - My boy builds coffins (original version)

Someone in the Dark by Michael Jackson

MJ recorded this for the film E.T., but it didn't make the soundtrack. It also then didn't make his Thriller album.

Hiding My Heart - Adele (from the 21 era). Only made a limited version of the album. Actually think it would of done well as a single, especially following the small promo it had when an amazing Scouser did a gorgeous cover of it.

The Killers - Under The Gun http://www.youtube.com/watch?v=3zOhYXAv1Jk
The Smiths - Asleep (It's been on compilations but it was a B-side) http://www.youtube.com/watch?v=-CGIii_eTOk

Alex and Bruno Mars- Before It Explodes
Leona- Scene of the Crime.

Kelly Clarkson - Chivas. BEAUTIFUL song

Aaliyah's 'Death Of A Playa'.That song was magical!It was included in a maxi cd single but that release was limited, so it is still under the unreleased category

Christina's 'Nasty' was a pretty great song and I think that if it was mastered, re-worked and re-recorded, it could fit in 'Back To Basics', probably the 2nd cd, next to 'Nasty Naughty Boy'

Beyonce's 'I Am Leaving' which is not strong enough for one of her albums, but I love it much more than her album tracks

And of course a lot of rappers do freestyles on clubs or on radio interviews and the more capable and iconic rappers always slay those freestyles and sometimes those little rap impromptus are so damn genius that I wish they were somehow released (not necessarily on an album) in high quality, the prime example being H to the OMO by Nas.