The highly-intelligent (and attractive) people over at the Water Testing Blog have posted some great resources to use if you're trying to find out exactly what's in your water:

Recently we have come across several useful web sites which feature water quality testing information, links to highly accurate home drinking water test kits from several reputable manufacturers, high quality water filter systems, and replacement filters for a wide range of drinking water filter systems. The links below will take you to their main pages:
Your Water Test
Quick Water Test
Water Test Blog
Air and Water Filters