Here are some sites that can be proven useful:
Kate Monk's Onomastikon. It provides a very detailed database with names across the entire world, proven hell useful for roleplaying; Middle Eastern, Oriental, European, Medieval, African, American, Pacific, and whatever else you can possibly imagine.
Vexilla Mundi, Flags of the World and World Flags provide a wide variety of world flags, historical or not, that can be proven VERY useful in role-playing. I have myself submitted the Cretan flag in Vexilla Mundi
The CIA World Factbook just rocks, doesn't it? All you need to know for any present-day country, including flags and maps. Another similar site is Index Mundi. Another one of them is the BBC News website, that has a country profiles section, where you can find out infomation about the country. On it is some information about the nation's media, which can be proven useful, especially since it links you to the offical sites of them.
Wikipedia is another great site for information about nations, all you have to do is type the name of the nation into the search box and click Go. You will be taken to a wikipedia page where it has all kind of information about that nation, it's history, a flag and coat of arms, the national motto, it will also have infomation about the politics of that nation. However, be aware that Wikipedia is user-edited, so it is not always reliable.
Foreign Office of the British Commonwealth. It has got alot of information in it, including boarding countries, politics, recent history and ecomony aswell as other information. Quite good and detailed.
PicSearch does not differ much from Google Image Search (Google in other countries: Greece, Belgium, Ireland, France, Germany, Russia, Netherlands, Spain, Portugal, Italy, Lithuania, Switzerland, Latvia, Finland, Sweden, Norway, United Kingdom.) Altavista is also a good site and image searcher.
Websites related to world armed forces that are useful to rp are Army Technology, Air Force Technology, Naval Technology, Haze Gray & Underway, the Federation of American Scientists (FAS) and Global Security. I myself use a lot the latter two ones for statistics, while the x-technology websites offer a variety of images for most modern military projects.
Babel Fish Translation and FreeTranslation are great online translators that can be used for your posts. FreeDict also has some languages that may not appear on the previous two such as Japanese and Indonesian.
Special Forces Units talks about the special forces of various nations; a useful site for rp'ers.
NationMaster offers statistics, maps, graphs, and other very useful stuff for the entire globe.
Thoughts? Post any useful sites worth to be added here, below. Also, report any broken links!