Last year I found out about Heart of the Alien
- the true successor to "Another World". In this game you control Buddy - your Alien friend from Another World. Lester, AW's hero, is a NPC in this game.
This game was only released on Sega Mega CD and can be played at full speed using notaz's great Picodrive-port for PSP.
I never got to play Flashback's (very different) successor on the PC - Fade to Black
. I found out there also was a PS1-version of it. The only difference between these versions is that the PC-version has simple gouraud-shading without textures while the PS1-version has (ugly) textures.
This game can be played perfectly emulated on PSP using Sony's PS1-emulator.
It even has been released in PSN so you could buy this game even if you have no custom firmware on your PSP. But for "Heart of the Alien" you definitely need custom firmware.
Eric Chahi also made Heart of Darkness
for PS1. It's similar to Another World and has good graphics and gameplay. This can also be played fullspeed on PSP.
P.S. Of course you can also play "Another World" and "Flashback" on PSP (through various emulators).