Here is a WIP list of Sega Saturn games and demos with Omake or Bonus(es) that can be found on their CD. It could be text files, pictures, sound files, screensavers or anything else.

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List of games with Omake / Bonus(es)

Serial	Region	Title	Omake / Bonus(es)
T-7611G		Air Management '96	5 .BMP files are readable.
T-7668G		Air Management '96 (Koei Best Collection)	5 .BMP files are readable.
MK81501-50		Baku Baku	README.TXT file contains some information (in Japanese). The european version of the game has an extra audio track.
GS-9040		Baku Baku Animal ~Sekai Shiikugakari Senshuken~	README.TXT file contains some information.
GS-9144		Baku Baku Animal ~Sekai Shiikugakari Senshuken~ (Satakore)	README.TXT file contains some information.
MK81803-50		Burning Rangers	18 .BMP files, 42 .WAV files and 1 .TXT file in the EXTRA directory.
T-2503G		Chou Aniki ~Kyuukyoku ... Otoko no Gyakushuu~	ONIISAMA.DOC file contains some information.
GS-9096		DecAthlete	README.TXT file contains some information.
T-27810G		Device Reign	DEV_ABS.TXT contains some information.
T-20102G		Doukyuusei ~if~	5 .BMP files in the CLASS.DAT file (renamed .lzh file). Inside the .lzh file, the OMAKE5.BMP file is in fact an Ish compressed file that contains 1 .BMP file and 1 .DOC file.
T-20104G		Doukyuusei 2	CLASS2.DAT file is an Ish compressed file where DQ2OMAKE.lzh file is stored. This .lzh file contains 17 .BMP files, 1 .WAV file, 1.DOC file and 1.TXT file.
T-13317G		Dragon Ball Z Idainaru Dragon Ball Densetsu (Satakore)	TF_DBZ.DOC file contains some information.
MK81382-50		Dragon Force	DISC_ABS.TXT file and DISC_BIB.TXT file contain some information.
T-19503G		Dragon Master Silk	42 .BMP files in the WALL directory.
T-10606G		FIFA Soccer 96	ENGLISH.TXT file and JAPANESE.TXT file are language files for the game and are readable.
GS-9101		Fighting Vipers	README.DOC file contains some information.
T-2111G		Game Basic for SegaSaturn	A lot of sample programs (.B files and other linked files).
T-5705G		Idol Janshi Suchie-Pai II	OMAKE.BMP file on CD1 but not readable on PC.
T-9528G		J.League Jikkyou Honoo no Striker	VS070_AB.TXT file and VS070_BI.TXT file contain some information.
T-2103G		Jikuu Tantei DD (Dracula Detective) ~Maboroshi no Lorelei~	31 .BMP files, 31 .TIF files and 1 .TXT file in the OMAKE directory of CD1. The DD_ABS.TXT file contains some information.
T-6607G		Madou Monogatari	11 .BMP files and 1 .TXT file in the OMAKE directory.
T-36302G		Maria ~Kimitachi ga Umareta Wake~	12 .BMP files in the EXTRA directory of CD2. Some of them seem corrupted or not in .bmp format.
T-23406G		MechWarrior 2	MW2.MAP file is the map file for the BCDFlat software (CD creation software)
GS-9127		Mr. Bones	MAPFILES.ZIP file in the 30MAP directory contains some .map compilation files of the game.
T-28901G		Mujintou Monogatari R ~Futari no Love Love Island~	ABS.TXT file contains some information.
T-20204G		Nightruth Explanation of the paranormal #01 "Yami no Tobira"	ABS.TXT file contains some information.
T-20203G		Nightruth Explanation of the Paranormal The Making of Nightruth	ABS.TXT file contains some information.
MK81020-50		Nights Into Dreams...	16 .BMP files in the EXTRA directory.
MK81048-50		Nights Into Dreams... + 3D Control Pad	16 .BMP files in the EXTRA directory.
T-10613G		Nissan Presents Over Drivin' GT-R	One MAIN.PCX file for each type of cars in the UIDATA directory.
T-10615G		Nissan Presents Over Drivin' GT-R (Premium Pack S-20 Engine Tokusei Key Holder-tsuki)	One MAIN.PCX file for each type of cars in the UIDATA directory.
T-5206G		Noon	NOON_ABS.TXT file contains some information.
MK81681-50		Photo CD Operating System	11 .PCD files in the IMAGES directory.
T-29301G		Pup-Breeder	PUP_ABS.TXT file contains some information.
GS-9169		Sakura Taisen 2 ~Kimi, Shinitamou Koto Nakare~ (Shokai Tokutenban)	SKRA_ABS.TXT file contains some information. SPC3321A.SN file in the SAKURA2 directory contains some pieces of code. All the CDs contain one CDDA track with the classic warning message said by different characters each time.
T-7617G		Sangokushi Returns	SAN1_CPY.TXT file contains some information.
MK81216-50		Sega Touring Car Championship	README.TXT file and RANKING.HTM file allowed you to register at the Global Ranking board (no more online).
T-10618G		Shin Theme Park	EAV_ABS.TXT file contains some information.
T-15018G		Taiheiyou no Arashi 2 ~Shippuu no Moudou~	AR2_ABS.TXT file contains some information.
T-15024G		Taiheiyou no Arashi 2 ~Shippuu no Moudou~ (Shokai Gentei Premium Box)	AR2_ABS.TXT file (CD1) and IMG_ABS.TXT file (CD2) contain some information.
T-7618G		Taikou Risshiden II	TR2LOGO.BMP file is readable.
T-7653G		Taikou Risshiden II (Premium Pack)	TR2LOGO.BMP file is readable.
T-25102G		Takuramakan ~Tonkou Denki~	TK_ABS.TXT contains some information.
T-7015H-50		Tilt!	12 .LBM files (Paint Amiga pictures) that can be viewed on your PC.
T-15911H-50		Titan Wars	You can watch the SECRET.AVI file to get this message: IRIS is dead!... But something far more terrifying and ugly is waiting for you on your final mission! Press RDDLCRAZY at the Pause screen
T-14401G		Touge King the Spirits	Special Halloween Atlus Logo (ATLUS.CPK file in the KAG directory).
GS-9060		Virtua Cop	2 files: THANKS.DOC file and OMAKE.DAT file which is in fact a lzh compressed file with 1 .JPG file and 1 .DOC file.
GS-9201		Virtua Cop 1 - 2 Pack	Virtua Cop: 2 files: THANKS.DOC file and OMAKE.DAT file which is in fact a lzh compressed file with 1 .JPG file and 1 .DOC file - Virtua Cop 2: README.DOC file contains some information.
GS-9097		Virtua Cop 2	README.DOC file contains some information.
GS-9154		Virtua Cop 2 Gentei Virtua Gun Set	README.DOC file contains some information.
GS-9059		Virtua Cop Special Pack	2 files: THANKS.DOC file and OMAKE.DAT file which is in fact a lzh compressed file with 1 .JPG file and 1 .DOC file.
GS-9180		Virtua Cop Special Pack (Virtua Cop 1 & 2 + The House of the Dead Taikenban)	Virtua Cop: 2 files: THANKS.DOC file and OMAKE.DAT file which is in fact a lzh compressed file with 1 .JPG file and 1 .DOC file - Virtua Cop 2: README.DOC file contains some information.
T-14304G		Virus	2 .JPG files and 1 .BMP file in the OMAKE directory of CD1.
GS-9177		Winter Heat	ABS.TXT file and SATFF.TXT file in the STAFF directory contain some information.
T-21401G		Zero4 Champ DooZy-J Type-R	Z4_ABS.TXT file contains some information.

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List of demos with Omake / Bonus(es)

Serial	Region	Title	Omake / Bonus(es)
610-6848		Baroque Report CD Data File	14 .BMP files in the OMAKE.LZH file and STNG_ABS.TXT contains some information.
MK81067-50		Christmas Nights Into Dreams...	4 .BMP files in the EXTRA directory.
610-6166-25		Flash Sega Saturn Vol.25	TENA_ABS.TXT file in the G002 directory contains some information about Tenant Wars.
610-6166-26		Flash Sega Saturn Vol.26	SKRA_ABS.TXT file in the G001 directory contains some information about Sakura Taisen 2 ~Kimi, Shinitamou Koto Nakare~.
610-6166-30		Flash Sega Saturn Vol.30	WAK_ABS.TXT file in the G001 directory contains some information about Waku Waku Monster.
610-6166-31		Flash Sega Saturn Vol.31	2 .BMP files and SMP_ABS.TXT file in the G002 directory. The .TXT file is the story of Lupin the 3rd ~Pyramid no Kenja~.