For some reason, every summer I get obsessed with card games. I think it links back to my days at summer camp—spending hours and hours in a platform tent playing Egyptian Rat Screw and BS. This summer I'll be spending in front of a computer, but I'll still get my card game fix!
Ancient Tripeaks—By far my favorite solitaire card game.
There are also a number of free online card games (you can find a bunch here) but they require registration.