I remember installing the demo of Outcast when it was released way back in 1999.
I had seen screenshots of it in magazines and it looked gorgeous. I had my hopes up. Did it look good? Yes. Did it look better than anything my eighteen year old mind could ever imagine? No. But it really fascinated me. As with Trespasser, I never got a chance to play the whole game, and that kept me wondering how it would be like. I wondered for a long time. Eight years, actually.
I tried Outcast yesterday, and I have to say I'm impressed. It certainly shows its age, but still has a graphical flair and special feel about it. The game is an even split of action and adventure (viewed from a first or third person perspective) and it's completely nonlinear in its structure - you can pretty much go anywhere at any point, and there are plenty of optional quests you may not even encounter unless you talk to every character you come across.
Outcast used voxels, a pixel technology developed and employed in games at the time. There's a more technical explanation of how that works here, but the short version is that voxels are efficient when it comes to rendering large outdoor landscapes without the use of polygons, allowing spectacular draw distance.
Unfortunately, Outcast isn't freeware, but it's available at your favorite Torrent search. There's also a demo, if that's more convenient (130 MB).