Saw this at target the other day. It claims to filter chlorine, mercury, lead, blah blah blah and filters the water through a mineral bed. I've got city water that's super soft and thought about giving this a whirl just to see if I could taste a difference in my beers.
You're not going to filter out any of the minerals that are important to brewing, or the alkalinity. A house water softener or a reverse osmosis system will, but not a charcoal filter, Pur, Brita, etc.