The cancellation of that game had brought  unspeakable  sadness into my soul at the time. It looked really cool.

Bioshock Vita is sad, as are the various versions of Star Wars Battlefront 3 that never saw the light of day. I love the first 2 but don't trust DICE not to turn it into yet another twitch shooter, which isn't really what the first 2 were like

Demonik.....still have a beta demo of it. Was really fun from what i was able to play.

Beyond Good and Evil 2

It may still be reborn and happen, but it was essentially canceled.

I can only remember a few.

Faith and a .45 on last gen consoles. Looked like a fun co-op romp with TF2 style cartoony graphics. Gunplay looked good. I think the developer ran out of monies.

Thrill Kill on PSone. I even had the PSone lid cover out of the PSM magazine on my PSone at the time. Fuck. Of course, the game turned out to be shit anyway.

Propeller Arena on Dreamcast. Canceled due to the 9/11 attacks. Silly SEGA, if you cancel the game, that means the terrorists win.

Star Wars 1313
Battlefront 3

Can't remember anything else right now.

Black & White port for PS1.

Timesplitters 4

Battlefront 3 (Free Radical's version)

Jak & Daxter PS3

Wild ARMs 6

Command & Conquer: Tiberium Incursion

-64DD
-Prey 2
-Sadness (Wii)

Omikron 2 - Nomad Soul is still one of my all time favorites, and I was pretty pissed when they postponed it and later canceled it in favor of Heavy Rain. Still hope that Cage will eventually revisit that world (and gameplay, while we're at it).

Project Offset, it looked great and it seemed like it has good story and gameplay, then Intel bought them to be showcase for Larrabee, once that died, game died too.

Off course, for me personally, nothing beats cancellation of Interplay's Fallout 3 (Van Buren) and Bethesda's acquiring Fallout IP.

Metroid Dread