Penny - pence - pennies
The word penny (a small coin) has two plural forms. One, pennies, is regular. The other is not - pence. As a rule of thumb, use the latter when you are giving a precise cost. When you are talking about the coins in general, use pennies - e.g. "I have a pocket full of pennies" but "The postage is twenty pence."
The position is sometimes complicated by the usages of those who learnt to handle money before the change to decimal coinage (see Pounds, shillings and pence). The use of an apparently singular form (penny) to express plural meanings can cause confusion. In such cases a number of pence (for example, a price) was expressed by the number followed by -penny, as a suffix, which formed an apparent noun acting as an adjective. The pre-decimal coin worth three (old) pence (valued at just over 1p in modern terms ("one New Penny")) was called a threepenny, pronounced (and sometimes written) as 'thruppenny' (in the North of England). The price was usually pronounced 'thruppence'. We also had things, be they ice creams, bus rides or cinema tickets called tuppenny (more formally twopenny), and in derogatory mood we could call something tuppenny-halfpenny, meaning 'very cheap'. The halfpenny itself was sometimes written ha'penny, and pronounced 'HAY-p'ny', IPA: /ˈheɪ pənɪ/.
In slang, if a man gave another a fourpenny one, it meant 'hit him very hard'. There has not been a fourpenny coin for a long time. In the olden days, there was one, called a groat; it was not issued after 1662, although a fourpenny coin (not called a groat) was issued from 1836 to 1856. (A fourpenny piece still forms part of the ceremonial Maundy money.)
You may also want to see Pounds, shillings and pence, which outlines the traditional British currency.