Games-on-demand service GameTap is offering S.T.A.L.K.E.R., Descent: Freespace, Deus Ex, Hitman: Blood Money and Psychonauts for free.
Just sign-up for the free GameTap client over on the website to begin. Our chums over at Rock, Paper, Shotgun note that the GameTap version of S.T.A.L.K.E.R. cannot be modified, which takes away an enormous part of the atmospheric PC shooter. Nevertheless, the free game can always be used as a demo.
RPS also points out that this may be a limited-time offer, so you'd better be nippy.
Head over to our S.T.A.L.K.E.R. review to see what we thought to years' ago. Remember, those mods make quite a difference.