My brother introduced me to
Proke, a free browser game where you race against the clock to type words and build a tower.
You're given a prefix and suffix, each three letters long. Use either one to make a word, and a tower is built on the screen. The more words you can use with the same prefix or suffix, the more points you get, and the taller your tower becomes. Every time your tower reaches a milestone height you get more time to play.
The game has 9 ranks, which are achieved by completing different objectives. They start out rather easy (use 5 of the bonus letters that appear periodically) but quickly become devious (use 15 words with the letters 'w' and 'k').
It's a fun game if you're linguistically inclined, and a great way to improve typing skills.