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]