The UK is experiencing a cold snap at the moment, with snow and ice affecting many areas (nothing as dramatic as the 'Polar Vortex' which some folks in the US Midwest are currently experiencing mind you!). In conditions like this, it's a good idea to make water available for wildlife wherever you can - and remember with sub-zero temperatures it's a process to repeat several times a day if you can.
If you have a pond, VERY GENTLY break the ice with a stick (go steady and try to avoid falling in - as I almost did!). Try to avoid pouring boiling water onto the ice as this can harm fish and other pond residents. Also, remember to ensure there's a ramp or means for any animals that might fall in to be able to climb back out again. Ideally it's best to build in a shallow end to your pond where possible.
As well as making water available to animals by breaking the ice, Toxic gases can build up in the water of a frozen pond, and these may kill any fish or frogs that are hibernating at the bottom.
Adding a tennis ball to the pond water can sometimes help prevent the surface of the water from freezing over completely. This only works if the ball keeps moving, so if the air is still and there's no wind or other reason for the ball to move, you may find this doesn't work - it's certainly worth trying though.
If you don't have a pond, it's important to provide water which is shallow enough for birds and other animals to be able to drink - unfortunately this does mean that the water will freeze more quickly than deeper water, so frequently changing it is even more important.
You may also find this page and downloadable PDF by the RSPCA interesting and helpful when caring for Hedgehogs and other visitors to your garden during the winter months.Hedgehogs should be hibernating between November and March so despite the decline in Hedgehog numbers don't be alarmed if you haven't seen one in your garden for a while - hopefully they're tucked up nice and warm somewhere safe.
Alternatively the BBC 'Discover Wildlife' Magazine suggests these '5 ways you can help wildlife this winter'.