Not all label-info available yet, we're also missing some singles but as far as we know every individual song is listed !

1979
LP
Bijou - Pas Dormir
Produced by Ron & Russell Mael

1979
LP
Noel - Is There More To Life Than Dancing
Written and produced by Ron & Russell Mael

1979
7"
Noel - Dancing Is Dangerous / I Want A Man
Written and produced by Ron & Russell Mael

1979
7"
Noel - Dancing Is Dangerous / Au Revoir
Written and produced by Ron & Russell Mael

1979
12"
Noel - Dancing Is Dangerous / The Night They Invented Love
Written and produced by Ron & Russell Mael

1979
7"
Adrian Munsey - C'est Sheep (part 1) / C'est Sheep (part 2)
Written and produced by Ron & Russell Mael

1979
12"
Sparks & Peter Cook - The No. 1 Song In Heaven / Tryouts For The Human Race
Buried deep in the run-off groove of the B-sides of the picture disc 12" singles of above tracks, are two, separate "comedy sketches" narrated by Peter Cook, about Sparks use of the Syndrum and it's place in world politics or some simular rubbish.
Info from Chris Devonshire-Ellis.

1981
LP/CD
Telex - Sex (Birds And Bees)
11 songs have lyrics written by Ron & Russell Mael, the track "Brainwash" has recently turned up in 2 new remixes on the Telex "I Still Don't Like Music" remix-compilation.
J'M2
J'M2-93029-200
Belgium

(info about the "Brainwash" remixes by Stephen Lennon)

1981
LP/CD
Ramones - Pleasant Dreams
Russell sings background vocals on this album.
Info from (a.o.) Chris Devonshire-Ellis.

1982
LP/CD
Lio - Suite Sixtine
Includes 6 songs with lyrics by Ron & Russell Mael
Wea
We 835
France

1984
LP/CD
Go Go's - Talk Show
It's not really a "cover" as Sparks never recorded it (as far as we know) but there is a song called "Yes or No" on this album credited to Wiedlin/Mael/Mael.

1987
CD
Salon Music - This Is
On this album, Russell appears as "special guest".

1988
CD
Salon Music - O Boy
On this album, Ron plays "african wood" (percussional instrument) on the track "In My Life", and Russell did put his vocals on the Soft Cell classic "Say Hello, Wave Goodbye".

1988
CD
Les Rita Mitsouko - Marc & Robert
Three songs recorded with Sparks: Hip Kit / Singing In The Shower / Live In Las Vegas
Virgin
CDV 2572
France

1988
7 inch  
Les Rita Mitsouko - Singing In The Shower
b/w Smog (non-Sparks)
Virgin
France
(info and pics from Michel Roman)

1988
12 inch  
Les Rita Mitsouko - Singing In The Shower (4:22)
b/w Smog (non-Sparks) / Hip Kit
Virgin
611861-213
France
(info and pics from H.F. Beiert)

1990
CD
Les Rita Mitsouko - Re
Two songs recorded with Sparks, remixed versions: Hip Kit / Singing In The Shower
Virgin
CDV 2637
France

1990
CD
Cheap Trick - Busted
Russell sings in the background on the song "You Drive I'll Steer"

1990
CD
Finitribe - An Unexpected Groovy Treat
Rumoured to be Russell's spoken L.A. accent on the track "Forevergreen".
Info from Chris Devonshire-Ellis.

1994
CD
Si-{cut}.db - "February Sleeper" / "Back In The Same"
(tracks on Just In Time For Too Late compilation)
Trivia: answering machine sample on "Back In The Same" is Russell Mael of Sparks.
Suburbs of Hell
SOH 016
UK

1995
CD
Duos - TARATATA
(Barclay, France, 1995, 529 509-2)
Contains a live version of Singing In The Shower together with Les Rita Mitsouko.
(info from Christophe Horlin)

1998
Afterword (written, not sung)
For the english translation of Serge Gainsbourg's one and only book "Evguenie Sokolov" (1980), Russell Mael was kind enough to write an afterword.
The book is published in 1998 by TamTam Books, Los Angeles, California.
ISBN 0966234618 (also available from amazon.com)
(info from Tosh Berman)

1999
Album track
Mulu is a new british band, according to Ron & Russell to be 'the next thing'. Laura Campbell of Mulu has written a song "David", and asked via BMG links if Russell would be interested in adding vocals (she had him in mind when she was writing it). He was very keen and sent a DAT of the vocals to her but they may meet up to do a live take in the future. The song was supposed to be released on Mulu's next album (planned release date: Oct. 1999), but has not seen the daylight yet.
Info from Gary Sanderson and Mulu's management.

2000
Album tracks
Grand Popo Football Club recorded two songs with Sparks on their first album "Shampoo Victims":
"Yo Quiero Mas Dinero" and
"La Nuit Est La".
Both tracks were co-written with the Maels, and you can hear Russell and Tammy singing.
Info from Benoit de Fleurac and Steve Boyce.

2000
Jacket blurb (written, not sung)
Ron Mael contributed a jacket blurb to Dr. Eckhard Nickel's latest book "Vas Ich Davon Holte" (2000). Dr. Eckhard Nickel seems to be a popular(?) German writer who interviewed Sparks on a number of occasions, and greatly impressed the guys.

2001
Album tracks
Sparks have collaborated with Japanese band "Pizzicato 5", on a track called "Kimono" (originally recorded by another Japanese group "TV Jesus"). The track can be found on Pizzicato 5's CD "Ça et là du Japon"
Also on this album is a hidden track, immediately after track two, which has Ron introduce himself and go into a brief, verbal thirty second tourist introduction to the geography of Japan, in English, after which the members of Pizzicato Five applaud. It is the longest that Ron has ever been known to say anything on record, although he does noticeably stuggle with pronouncing some of the Japanese placenames !
Info from Chris Devonshire-Ellis

2002
Marc Moulin - Top Secret (2CD)
Ron and Russell have contributed to Telex' Marc Moulin's album (2CD's) "Top Secret". Ron contributed his vocals on the "What?" track, Russell engineered.
More info here.
(info from Magnus Kaukerat)