These were the ones from the top five on http://www.scene.org in the releases area for Assembly 07 - a long standing oldskool demo party. Demo parties are where all the worlds' best 3d coders go to show off their talents in competition.
The demos I link below require a lot of processor - try them on your best machine and see how they go.. if not, http://www.scene.org is a great place to find demos that will really test the limits of your pc - whatever age -- demo parties have been going for decades and all the releases are at that site = http://www.scene.org (also mac.scene.org for those inclined).
ADHDTV recommended > 1GHz CPU
Continuum recommended > 1GHz CPU
pangea_ultima recommended > 1GHz CPU
I deliberately repeat that speed recommendation.. they aint gonna run without it... the 3D in them is rendered in realtime using your 3d graphics card.