Word Squares
This is a fun game for any number of players. Except one. I guess you could play it solo, but then it's kinda against the point.
The general idea is this: each player draws up his own 5 x 5 grid on a piece of paper. Players keep their own grids hidden from one another. The game starts with one player calling out a letter, which must be placed somewhere on the grid. The game continues with the next player in turn calling out another letter. Players attempt to form words on their grids.
The game continues until 25 letters have been called out, at which time play is stopped, and players score their grid. Score 2 points for a 3-letter word, 5 points for a 4-letter word, and 10 points for a 5-letter word. Naturally, the player with the highest points total is the winner.