Deus Ex: Human Revolution has had a leaked preview build available online for a few weeks now, letting people try the game out ahead of its release.
It's not the whole game - just the opening sections - and as such, publishers Square Enix have been seemingly cool with people downloading it and giving it a try. If you lack the will or means to do so on a PC, here's someone playing through the first forty minutes of the game, recorded for your viewing pleasure.
Note that this is from a preview build, and not the final game (which isn't due until August), so if something looks a little janky, that may be why.
Walkthrough [Super Punch]