This map will be of interest for gardeners in Europe as it gives the highest and lowest annual minimum temperatures. If you want to see more detail for your country click http://www.gardenweb.com/zones/europe/
Here is an example of the detailed climate zone map for the UK. In reality, the frequency of -12 in SE England is very rare. Most winters (2008/9 being the exception ) the temperature rarely dips below -7 in most areas of the South East. Plus some gardens have their own micro-climates where temperatures will be consistently warmer.
It will be interesting to see how this climate zone map changes over the next 50 years as weather patterns change and warmer temperatures travel north.