I usually don't do anything to it, although I probably should be adding a little calcium, since my water is deficient in that. Eventually I'll get to tuning my brewing water to be style-appropriate, but I figure that's overkill while extract still makes up well more than 50% of my fermentables.
I do boil my top-off water for half an hour to get rid of the chlorine, let it cool and fill it into 1-gallon jugs to chill in the fridge overnight.
And sometimes I'll just get bottled spring water if I feel like being lazy.
The Fiesty(sic) Goat Brewery est. 2007 & Clusterfuggle Experimental Ales est. 2009
Primary: Saison, Berliner Weisse, sLambic I, sLambic II, IPA, 70/-
Secondary: Orange Blossom Mead, Flanders Red I
Kegged: Fat Man Porter w/ 1469, Fat Man Porter w/ 005, Centennial Falcon w/ Conan, Centennial Falcon w/ Denny's, Barrel-aged Fat Man Porter, Belgian Dark Strong, Dark Mild