I know it may not feel like it right now, but soon - please, oh please - it will be summer, and if you travel on public transport or go for a walk, you'll be advised to take your water bottle with you. A few years ago in Australia I bought a Brita Water Bottle that I took everywhere with me. It was really good, so I'm pretty pleased to see these on Groupon.
These are 600ml bottles, which fit conveniently into standard cup holders. They are dishwasher safe and are designed to remove impurities such as chlorine.
What is really good with this offer is that they come with four filter discs. You have the option to increase that to 12 or 20 though, which is great. When I brought mine back to England, I couldn't find new filters, so it was resigned to the cupboard, which is less great.
The bottles are nice and streamlined too, so they look quite nice.
Just a heads up though - you have to pay an additional £1.99 for postage.