Bookworm is my favorite Popcap game, even though I haven't played them all. It's another game that can run long but is easy to walk away from when necessary.
It's a word game with simple rules: just trace a path around the game board to spell a word. Those tiles are then removed and new tiles drop in to take their place. Longer words get you more points. The challenge increases as your score gets higher, as the game will drop burning tiles that will eat their way down to the bottom of the screen and end the game if you don't spell a word with them. If you make a lot of short words, you get more burning tiles. Conversely, spelling longer words will get you green, golden, and crystal tiles that give big bonuses when used. The game also starts picking random words for you to try and spell for an increasing amount of bonus points.
The game is great for a quick distraction or extended play. In the standard mode of the game, there's no time limit for spelling a word, so it's very casual. Best of all, you can play it for free on the web. The $19.95 downloadable version does add some nice features though, including the ability to save and quit, and both an internet and local high score list (which keeps track of both scores and longest words spelled).
Read [Bookworm Download Page]
Play [Bookworm Free Online Version]