4. Tetris
Though not quite as easy to obtain as Solitaire, there are so many online versions of Tetris that it's not hard to track down as long as you have an Internet connection. It's another game that's exceedingly simple, and that's the key - the game requires the player to fit together blocks of shapes by colour, and when you do this, the blocks disappear. However, it requires quick wits to go far, because soon enough, the blocks will end up filling the screen, and the only "end game" is when a block touches the ceiling. What makes the game addictive is the ever-troublesome high score; trying to top your previous best is a transfixing experience that quickly becomes addictive. For a game without the faintest scrap of a story or narrative, that's impressive; it's all about the basic genius of the concept, and how gratifying it is when you manage to come out on top (much like a fruit machine, I expect).