The WORST invention of the 20th century (& 21st) - (Alternative title - Scrooges guide to technology).
My nominations are the mobile phone, and twitter.
Mobile phones, whilst useful, have become a total pain with idiots wandering around frying what passes for their brains by having them permanently glued to their ears and annoying ring tones.
As for twitter - aptly named when you see all these twits constantly apologising for their stupid comments. What actual purpose does it serve ?