Cancelled, yes...but it's not exactly unavailable. I could chuck my copy in my Dreamcast right now...I hope you knew this, and are upset because you never got to play it online, rather than the much worse possibility that you've been sitting there for thirteen years not knowing you can just download it off the internet! It's a fucking blast in 4 player split screen.
Stalker 2, Timesplitters 4, Starcraft Ghost, and so many others in this thread that I came to say. But how about the game that was cancelled, so that the developers could take over work on Starcraft Ghost?
Metal Arms 2
The original game, Metal Arms: Glitch in the System is easily one of my favourite games ever made, and the only game the developer, Swingin' Ape Studios, ever got to make. The whole story, from trying to create a bounty hunter game, licencing their own engine out to survive, to being ranked the best game Vivendi had at E3 and the cancelled sequel, is in this great article
here.
What it doesn't include is what happened afterwards, when the whole team was bought out by Blizzard, and put to work on Starcraft Ghost, which was at that point in time a complete mess under the direction of Nihilistic Software - who sucked then, and suck more now. It's pretty fascinating, especially if you played Metal Arms, to see Starcraft Ghost suddenly (and obviously) switch on to the Metal Arms engine, and the Swingin' Ape team make their mark on the game. And then boom. Cancelled.