Quite many times you will to check something of investigate something further. The following links are for sites and online services I have found handy and easy to use. Feel free to recommend something that should be listed in your opinion, living is learning as the saying goes:

Information about different countries

Wikipedia - maybe the most reliable source of country information as it tends to be pretty much up-to-date and nobody profits by twisting the thruth.
Wikitravel - free worldwide travel guides
Embassyworld - find out where your embassy is located in each country
Lonely Planet - good source of information also when planning regular trips

Currencies etc.

Oanda - currency converting made easy
Worldclock - check the time before making that call to your relatives

Traveling

OnlineAuto - easy way to compare all the biggest chains and hire your rental car from anywhere
Expedia - flight tickets for a bit cheaper