My husband and I use of Angie's List--at least in terms of finding a contractor. For big jobs, we get multiple quotes. We just went through this with replacement windows.

The Internet is your friend as well. It's rare that I can't learn what I need to learn online. It does take a lot of digging and some patience, however, to sift through all the info. There are forums on just about every topic under the sun, buying guides, reviews, etc. Consumer Reports can be helpful, too, depending on the topic. They offer online subscriptions.

Just in case you hadn't already seen it, here's a buying guide for tankless water heaters:
http://www.tanklesswaterheaterguide.com/

In researching replacement windows, I came across the gardenweb.com forums. Notwithstanding the name, there are subforums specific to the home, including one on plumbing. You might check there.