Economists call it a positive externality, we just call it cool. Lonely Planet, the accredited travel and information guide, is offering thirteen of its city guides for free on the App Store due to the Icelandic volcano. The applications, which offer you offline maps, geo-coded points of interest and location-based navigation, usually cost between $10 and $15. Hurry, the offer ends on 22nd April and we can be pretty sure that it won't happen again soon. Then again, none of us are volcanologists.