Words With Friends
"Words With Friends" earns its place in this list because for many people, it's very addictive to prove just how clever you are to your inferior friends. "Words With Friends" is essentially competitive Scrabble, but its asynchronous gameplay makes it easy to have multiple matches running at the same time.
Players take turns placing letters on a board filled with bonus tiles (like double letter, triple word, etc.) and can go hours or days between plays if they so choose. Designer Zynga, the company that's also behind the wildly popular "Farmville," knows all about creating addictive experiences. "Words With Friends" may play at a leisurely pace, but don't let that fool you -- you'll be embroiled in ferocious competition before long.
"Words With Friends HD" costs $2.99 in the iTunes App Store as of August 2011.