Today's XKCD really is wonderful: called 'Now', it's a simple clock which shows you what time it is in the world right... now, and changes each time you check back on it.
It's created in such a way that it's simple to read. Noon and midnight always sit in the same place and each time you visit, the place names and maps of the world will have shifted to show you what time it is, everywhere, right now. Check it out. [XKCD]