Treasure was formed in 1992 by a group of game developers who left Konami. Before leaving, they were responsible for developing the Contra series and Castlevania among others. Their first release was Gunstar Heroes in 1993. Below is a list of all their releases with publishers and release dates sorted by platform.

Radiant Silvergun
Treasure 5/1998 Japan (Sega ST-V Titan)
Treasure 12/20/2001 Japan (Sega Naomi)
Bakuretsu Muteki Bangaioh (Bangai-O in US and Europe)
ESP 12/9/1999 Japan
Swing! 10/20/2000 Europe
Electro Source 3/21/2001 US
ESP 9/5/2002 Japan
Game Boy Advance:
Tiny Toon Adventures: Buster's Bad Dream
Swing! 7/5/2002 Europe
Conspiracy TBA US
Hajime no Ippo: The Fighting
ESP 12/12/2002 Japan
Infogrames 1/16/2003 Japan
Infogrames 4/16/2003 US
Infogrames TBA Europe
Mega Drive/Genesis:
Gunstar Heroes
Sega 9/10/1993 US and JP
McDonald's Treasureland Adventure
Sega 9/23/1993 Japan
Sega 12/1993 US
Dynamite Headdy
Sega 8/5/1994 Japan
Sega 6/14/1994 US
Yuu Yuu Hakusho
Sega 9/30/1994 Japan
Alien Soldier
Sega 2/24/1995 Japan
Light Crusader
Sega 5/26/1995 US and JP
Nintendo 64:
Bakuretsu Muteki Bangaio
ESP 9/3/1999 Japan
Yuke Yuke! Trouble Makers (Mischief Makers in US)
Enix 6/27/1997 Japan
Nintendo 9/30/1997 US
Tsumi to Batsu: Chikyu no Keishousha (Sin and Punishment: Successor to the Earth)
Nintendo 11/21/2000 Japan
Rakugaki Showtime
Enix 7/29/1999 Japan
Silhouette Mirage
ESP 7/23/1998 Japan
Working Designs 1/6/2000 US
PlayStation 2:
Silpheed: The Lost Planet
Capcom 9/21/2000 Japan (re-released 9/26/2002)
Working Designs 5/1/2001 US
Swing! 5/11/2001 Europe (re-released 7/19/2002)
Stretch Panic (Freak Out in Europe, Hippa Linda in Japan)
Swing! 7/27/2001 Europe
Conspiracy 8/30/2001 US
Kadokawa Shoten 12/6/2001 Japan
Tiny Toons Adventures: Defenders of the Looniverse
Conspiracy TBA US
Infogrames TBA Europe
Guardian Heroes
Sega of Japan 1/26/1996 Japan
Sega 4/24/1996 US
Silhouette Mirage
ESP 9/11/1997 Japan (re-release 12/10/1998)
Radiant Silvergun
ESP 7/23/1998 Japan

There is no doubt that there were numerous cancelled titles as well. One high profile project they had was Magical Hearts: The GunBeat (aka Gunbeat.) It was a racing/shooting game being developed for Sega's Naomi arcade hardware. The project was officially cancelled 5/22/2000.

I had difficulty confirming the existence of some games, the Game Gear versions of Dynamite Headdy and Gunstar Heroes in particular. I'm not sure if these were developed by Treasure or re-coded by another company. I also believe that the Yuu Yuu Hakusho series for SNES was not developed by Treasure. If you believe that any of this information is inaccurate, please /msg me.
I've redone the formatting and decided to keep all the publishers hardlinked for readability. Feedback on this is appreciated.