TypeRacer. Race other people at typing a few sentences, with car icons showing who’s leading. After a few races you get ranked, and then only play people of a similar skill level. Annoying as hell, but totally addictive. My highest so far is 81wpm – I’m not bad at touch-typing, but the deletions always go wrong.
Hint: it’s probably best to close the windows if your neighbours object to swearing. Via @heilemann.