I have become addicted to Yahoo's Bejeweled 2. I am constantly playing the free web version while I work the computer. I prefer the classic, untimed version, where you need to link jewels to form chains that are three jewels or longer. Four jewel-long chains create a bomb, which explodes the next time you use that jewel to form a chain. Chains with five jewels form a hyper sphere, which eliminates all the jewels of the color of your choice.
When I get bored of this game, I turn to Super Text Twist. You have two minutes to form as many words as possible using the supplied letters. You need to find the longest word possible to proceed to the next level.
I also enjoy Bookworm. You link letters on books to form words. If you form too many three letter words, a burning book will appear on the screen. You will have to use the letter on this book to form a word before it reaches the bottom of the stack or the fire spreads to other books. If a burning book reaches the bottom of the stack, the library burns down.
What are your favorite online games?