(IPA notation; HTML character entity/Unicode character ŋ, in ASCII IPA and many other systems represented as "N")
Also known as "eng" sound; the proper technical name for this is velar nasal.
For most speakers of western languages, I've noticed, this is one of the hardest because it's most often not written as ŋ. (Then again, with languages like English, mostly everything is not written as pronounced so this is one of the least problems when learning to read and write =) In Finnish, ŋ is written as "nk" ("kenkä", "hanki") and double-ŋ as "ng" ("kangas", "vangit"). (Speaking of "vangit", I recently read a fan page of a game - the writer consistently spoke of "vankit". Obviously, this sound is still pretty hard to spell.) In Finnish, I've noticed that "nk" often transforms into "ng" when the word is pluralized ("kenkä" -> "kengät").
Personally, I had first problems trying to write a penguin's name (fitting start for a Linux user =): I had problems writing "Pingu". No matter how hard I tried, it turned out to be "piggu" (which, read aloud, sounded completely different, of course...)