Over in Ireland, c is used for cent and euro amounts are written as eg. €25.99. The Irish post office issued a stamp commemorating the euro back in 1999 and it was denominated as 30p (pence) and 38c and all (main value)stamps since late 2000 have been dual-denominated and 'c' was always used as the abbreviation, so I guess everyone else just followed suit after that!
In the newspapers, 'c' is normally used, although you'll occasinally see the 'American' ¢ (cent) version as well. Supermarkets, etc. use 'c' too, assuming the item is cheap enough
As for spellings, euro and cent are usually spelled the same in singular and plural, though sometimes euros and cents are sometimes seen.
