www.gametap.com
actually signed up for a paid version of this for my son but there are loads of games on there for free and they regularly add games to the free list for a week or so with stalker and battle stations midway/pacific being some of the recent ones
http://gametapblog.com/free-game-of-...r-fun-edition/
forgot to add, they have loads of browser based games including some older stuff